Adsorption thermal energy storage works by exploiting the fact that water molecules bind strongly to the enormous internal surface area of microporous solids such as zeolites. When humid air flows over a dry zeolite bed, water vapour is taken up and the heat released during binding can be delivered to a process or a building. Charging the store reverses the process: heating the bed drives the water out, and the energy is effectively locked in the desiccated crystal until needed. Because the energy is stored chemically rather than as sensible heat, storage densities of around 180 kilowatt hours per cubic metre are theoretically achievable, several times what a hot water tank can manage. Open adsorption systems have already found commercial niches, notably in an energy-efficient dishwasher developed by Bosch Siemens Hausgeräte together with ZAE Bayern, but researchers in the field concede that adsorption heat storage remains in the early stages of commercialisation.

The central problem the German team set out to solve is one of matching. An adsorbent that performs superbly at one combination of temperature and humidity may be mediocre at another, and the conditions are dictated entirely by the application. A mobile storage unit harvesting industrial waste heat operates under very different temperatures than a sorption system built into a tumble dryer. The team therefore proposed a method in four steps: first compare candidate materials under application conditions using characteristic curves, then measure dynamic breakthrough curves in a realistic fixed bed, third validate a computer simulation of that bed against the experiments, and finally test the hydrothermal stability of the shortlisted materials by cycling them hundreds of times under the actual operating conditions. The novelty, the authors argue, lies in folding hydrothermal stability testing, which previous studies often omitted, into the design workflow itself.

The first step hinged on measuring adsorption equilibria with unusual precision. The team used a simultaneous thermal analysis instrument operated in thermogravimetric analysis mode, coupled to a humidity generator and a high-accuracy dew point mirror calibrated over a range from minus 50 to plus 95 degrees Celsius with an uncertainty of no more than plus or minus 0.1 degrees. Roughly 300 milligrams of each zeolite were exposed to carefully controlled combinations of temperature and dew point, generating the equilibrium data needed to construct so-called characteristic curves. These curves, rooted in adsorption potential theory going back to Dubinin, plot the volume of water adsorbed against the adsorption potential, a measure of the free energy change as vapour binds to the pore walls. Once a characteristic curve is known, the volumetric water uptake for any pair of adsorption and desorption conditions can be read off directly, allowing materials to be compared before a single prototype is built.

The measurements proved reliable. For the 13X material the characteristic curve was valid across adsorption potentials from 105 to 3,495 kilojoules per kilogram, and for the NaY material from 106 to 3,524 kilojoules per kilogram, with combined measurement uncertainties of only a few kilojoules per kilogram and about 0.001 cubic centimetres per gram in adsorbed volume. Crucially, adsorption and desorption measurements coincided almost perfectly, indicating that true equilibrium was reached and that neither material shows meaningful hysteresis. The curves also agreed closely with published data for the same and similar zeolites, confirming that the thermal analysis route can replace bulkier methods that require nearly 50 grams of sample. When the team then read the curves against two application scenarios, a mobile sorption storage and an industrial tumble dryer, the NaY zeolite came out slightly ahead in both cases, delivering a volumetric water turnover of 0.270 cubic centimetres per gram for the mobile scenario compared with 0.240 for the 13X, and 0.266 against 0.235 for the dryer scenario.

Static equilibrium, however, says nothing about how fast a real storage bed charges and discharges. For that, the researchers built an open-air test rig in which a blower pushes air through a countercurrent humidification column and an electric heater capable of reaching 350 degrees Celsius, before the flow enters a fixed bed of zeolite pellets. The column, 148 millimetres in diameter and filled to a height of 300 millimetres, is double-walled with evacuated perlite filling to form vacuum super insulation, minimising heat losses and temperature gradients across the bed. Dew point mirrors and Pt100 sensors continuously recorded the air conditions immediately before and after the bed. The resulting breakthrough curves, which track how temperature and humidity at the outlet evolve over time, revealed clear differences between the materials. During desorption, the NaY bed showed sharper, more pronounced transitions, which are generally preferred in applications, whereas during adsorption the 13X bed changed state more abruptly and cleanly.

These experimental curves then served as the benchmark for a one-dimensional fixed bed simulation built on four coupled partial differential equations describing the mass and energy balances of gas and sorbent. The model treats the pellets as isothermal, accounts only for mass transfer resistance in the gas boundary layer around the grains, and was run in Matlab with spatial steps of 0.25 to 0.5 centimetres and time steps of one to two minutes. When adjusted for heat losses, the simulation reproduced temperature levels and phase durations of the measured breakthrough curves very well, and even captured a striking dew point peak in the first hour of NaY adsorption runs, confirming that this behaviour was genuine start-up dynamics rather than a measurement artefact. Transitions in the simulation were somewhat sharper than in the experiments, a discrepancy attributed to airflow maldistribution and thermal capacitances not covered by the model. With validation complete, the tool can now be used to scale column geometry and operating parameters to real systems without repeating costly experiments.

The final and, in this case, decisive step was the cycling test. A purpose-built rig subjected both zeolites to 140 full cycles under conditions derived from the tumble dryer integration: desorption at 300 degrees Celsius with a dew point of 60 degrees, and adsorption at 42.5 degrees Celsius with a dew point of 30 degrees. Reference cycles at milder conditions were run periodically, and the samples were weighed to track water uptake. The 13X zeolite began with an adsorbed water mass of 0.281 grams per gram of dry material, but lost 4.6 percent within just ten cycles and finished at 36.5 percent below its starting capacity after 140 cycles, consistent with earlier reports that zeolite 13X loses roughly half its capacity when exposed to 300-degree humid conditions. The NaY zeolite, starting slightly higher at 0.292 grams per gram, lost only 1.1 percent after ten cycles and just 3.3 percent after 140, with the rate of decline slowing as cycling continued.

For the envisioned application the conclusion is unambiguous: 13XBFK should not be used under those conditions, while NaYBFK shows the hydrothermal resilience needed to sustain storage capacity over a service life. The team is careful to note that stability verdicts cannot be transferred between applications, since lower or higher temperatures and dew points change the ageing chemistry, and that economics and adsorption enthalpies, which feed into the coefficient of performance, must also enter the final decision. The authors acknowledge that their methodology is time-consuming and demands up to three separate test setups, a drawback they consider nonetheless essential for responsible sorption system design. The work was part of the AMThES project supported by the German Federal Ministry for Economic Affairs and Climate Action.

The broader significance of the study goes beyond one dryer or one storage vessel. It demonstrates a template for screening adsorbents in a way that mirrors reality: equilibrium data first, dynamics second, validated simulation third, and brutal long-term stability testing last. As industrial waste heat recovery and electrified heating push the demand for compact, high-density thermal storage, the difference between a material that fades by a few percent and one that collapses by a third could determine whether adsorption technology finally moves from the laboratory bench to the factory floor. For zeolite NaY, at least under these punishing conditions, the future looks considerably drier, and that is precisely why it lasts.

Subject of Research: Application-oriented characterisation and simulation of zeolite materials for adsorption thermal energy storage

Article Title: Application oriented material characterisation and simulation for adsorption thermal energy storage

Article References: Schmit, H., Pöllinger, S., Schubert, T., Lävemann, E., & Hiebler, S. (2025). Application oriented material characterisation and simulation for adsorption thermal energy storage. Frontiers in Thermal Engineering, 5, Article 1513507. https://doi.org/10.3389/fther.2025.1513507

Image Credits: AI Generated

DOI: 10.3389/fther.2025.1513507

Keywords: adsorption thermal energy storage, zeolite, characteristic curve, breakthrough curves, fixed bed simulation, hydrothermal stability, thermogravimetric analysis, water adsorption, thermal energy density, tumble dryer, mobile sorption storage, cycling stability

The study, published in the Journal of Saudi Chemical Society, describes ZIF-8/CA composite porous carbons: materials built by combining zeolitic imidazolate framework-8, a metal-organic framework prized for its ultrahigh surface area, with cellulose acetate, the biocompatible and biodegradable polymer that already dominates commercial cigarette filter manufacturing. The research was led by Wei-li Xu, Pei-jian Sun and Cong Nie of the Key Laboratory of Tobacco Chemistry at the Zhengzhou Tobacco Research Institute of CNTC, working with colleagues from China Tobacco Shaanxi Industrial and China Tobacco Shandong Industrial. Their central insight is deceptively simple: the ratio in which the two ingredients are mixed before carbonization decides everything about how well the final carbon can grab benzene molecules from a moving gas stream.

To build the composites, the team turned to a dual emulsion-solvent evaporation method, a technique borrowed from soft-matter chemistry that is rarely applied to metal-organic framework composites. First they synthesized ZIF-8 powder by mixing zinc nitrate hexahydrate with 2-methylimidazole in water at a precisely controlled molar ratio and stirring the milky suspension for 24 hours. They then created a primary emulsion by injecting an aqueous internal phase containing ammonium bicarbonate into a solution of cellulose acetate dissolved in dichloromethane, adjusting the ZIF-8 to cellulose acetate mass ratios to 3/7, 4/6, 6/4 and 7/3. Droplets of this emulsion were dispersed into a dilute polyvinyl alcohol solution to form a secondary emulsion. As the solvent evaporated over six hours of stirring, uniform composite microspheres formed. The final and most transformative step was pyrolysis: heating the microspheres to 950 degrees Celsius at 5 degrees per minute under nitrogen and holding them there for two hours, converting the polymer-framework hybrid into porous carbon.

Why go to such lengths? The answer lies in the complementary weaknesses of existing adsorbents. Conventional activated carbon, the workhorse of volatile organic compound control, suffers from a moderate specific surface area, irregular pore structures and weak affinity for benzene at low concentrations. ZIF-8, by contrast, offers exceptional microporosity, strong pi-pi interactions with aromatic molecules and a gate-opening flexibility that accommodates bulky guests like benzene and toluene — but the raw powder is notoriously difficult to process, tends to agglomerate, and cannot easily be shaped into practical devices such as filter cartridges. Cellulose acetate solves the processability problem: its hydroxyl and acetyl groups form hydrogen bonds with ZIF-8, promoting even dispersion and mechanical stability, while its carbonization generates additional pores. What remained unknown, and what this study set out to map systematically, was how the ZIF-to-polymer mass ratio shapes the full hierarchy of pore sizes and, in turn, the benzene uptake.

The characterization data tell a striking story of structural optimization. Scanning electron microscopy revealed spherical particles between 100 and 500 micrometers in diameter, but their internal architecture varied dramatically with composition. At the 3/7 ratio, excess cellulose acetate caused the polymer matrix to aggregate, producing a dense, nearly pore-free cross-section. Pushing ZIF-8 content too high, at 6/4 and 7/3, triggered the opposite failure: agglomerated carbon particles and larger but poorly distributed pores. The sweet spot arrived at 4/6, where the surface roughened into uniform microscale protrusions and the cross-section displayed a beautifully interconnected porous network. Nitrogen physisorption measurements confirmed the visual evidence: the 4/6 composite achieved a Brunauer-Emmett-Teller specific surface area of 1380 square meters per gram and a mesopore volume of 0.77 cubic centimeters per gram, far exceeding the 577, 657 and 796 square meters per gram recorded for the other formulations. All samples showed Type IV isotherms with H4 hysteresis, a fingerprint of mesoporosity, with mesopores concentrated near 2.5 nanometers.

Mercury intrusion porosimetry added the final piece of the hierarchical puzzle, probing pores far too large for nitrogen adsorption to detect. The dominant macropore diameter increased progressively with ZIF-8 content, a trend the researchers attribute to gas evolution — carbon dioxide and nitrogen released as the framework decomposes — which inflates and expands the pores during carbonization. This revealed an elegant division of labor: cellulose acetate carbonization builds mesopores and surface area, while ZIF-8 decomposition carves macropores. The two effects compete, and the 4/6 composition strikes the optimal balance between abundant mesoporous adsorption domains and macroporous highways for rapid molecular transport. Many high-surface-area carbons perform poorly in dynamic adsorption because blocked channels or excessive micropores strangle diffusion; this composite avoids both traps by design.

Dynamic benzene vapor adsorption tests brought the structure-property relationship into sharp focus. In breakthrough experiments with a benzene stream flowing at 100 milliliters per minute through a thermostatted column at 25 degrees Celsius, the saturation adsorption capacity traced a volcano-shaped curve across the composition series, peaking at 235.0 milligrams per gram for the 4/6 sample — well above its siblings and competitive with previously reported adsorbents. To dissect the kinetics, the team fitted the breakthrough curves with two classical fixed-bed models. The Apiratikul-Chu model reproduced the entire breakthrough curves with correlation coefficients exceeding 0.99, capturing the asymmetric tailing that arises from internal diffusion through tortuous pore networks. The 4/6 composite also displayed the highest rate constant, at 65.2 per minute, thanks to plentiful active sites and macropores that slash mass-transfer resistance. The Adams-Bohart model, applied to the initial stage of adsorption, delivered correlation coefficients above 0.97, confirming that surface adsorption and external mass transfer govern the onset of uptake.

The chemistry behind the capture is as important as the physics of the pores. Both the graphitic carbon formed from cellulose acetate and the residual framework structure of ZIF-8 are rich in delocalized pi-electrons, which form strong pi-pi stacking interactions with the aromatic ring of benzene — an affinity that physical adsorption alone cannot provide. Meanwhile, the graded pore hierarchy produces a confinement effect: micropores and mesopores physically trap benzene molecules, while interconnected channels accelerate their diffusion toward those sites. Macropores lower external diffusion resistance, mesopores and micropores dominate intraparticle diffusion, and the result is a material whose adsorption rate is as impressive as its capacity.

The most headline-grabbing result came when the material faced its intended real-world challenge: actual cigarette smoke. Using a custom-built apparatus connected to an SM-450 smoking machine operating under the ISO standard protocol — 35-milliliter puffs drawn over two seconds with 60-second intervals — the researchers loaded 10 milligrams of adsorbent into each cigarette filter and analyzed trapped benzene by gas chromatography-mass spectrometry. The ZIF-8/CA 4/6 composite removed 49.0 percent of benzene from mainstream smoke, dramatically outperforming a ZIF-8/polylactic acid control composite prepared under identical conditions, which managed only 20.7 percent. Even in the chemical chaos of real smoke, where countless gaseous components compete for adsorption sites, the hierarchical pore structure and pi-pi selectivity for aromatic compounds allowed the composite to maintain high benzene capture. The authors note that only benzene was quantified in this study, with multi-component analysis and regeneration and long-term cycling tests planned for follow-up work, and that static adsorption isotherms will be examined in future research.

Beyond the laboratory numbers, the study delivers a genuinely versatile design principle. By simply tuning a mixing ratio before a single carbonization step, researchers can dial in a micro-meso-macroporous architecture tailored to a target pollutant — a rational, adaptable strategy that could extend well beyond cigarette filters to industrial off-gas treatment, indoor air purifiers and protective respirators. The rigid carbon skeleton also promises structural stability for cyclic use. For a compound as pervasive and as dangerous as benzene, a scalable material that more than doubles the removal efficiency of a comparable commercial polymer composite represents meaningful progress — and a reminder that sometimes the biggest advances in environmental health come not from exotic new chemistry, but from getting the architecture of familiar ingredients exactly right.

Subject of Research: Hierarchical pore engineering of ZIF-8/cellulose acetate composite porous carbon for benzene vapor adsorption and cigarette smoke purification.

Article Title: Hierarchical pore structure modulation of ZIF-8/CA composite porous carbon for efficient benzene vapor adsorption

Article References: Xu, W.-L., Sun, P.-J., Sun, X.-H., Wang, Y.-P., Li, J.-L., Ge, C., Liu, Q., Zhou, J., Yang, F., Song, X.-H., & Nie, C. (2026). Hierarchical pore structure modulation of ZIF-8/CA composite porous carbon for efficient benzene vapor adsorption. Journal of Saudi Chemical Society, 30(4), Article 56. https://doi.org/10.1007/s44442-026-00098-2

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00098-2

Keywords: ZIF-8, cellulose acetate, porous carbon, benzene adsorption, volatile organic compounds, hierarchical pores, metal-organic frameworks, cigarette smoke filtration, air purification, carbonization, adsorption kinetics, breakthrough curves