Experimental Study on the Kinetics of CO2 and H2O Adsorption on Honeycomb Carbon Monoliths under Cement Flue Gas Conditions

The main challenge of adsorption consists in the production of materials that can be used in real situations. This study comprehensively describes the CO2 and H2O adsorption behavior of honeycomb-shaped sorbents commonly used in rapid pressure swing adsorption cycles (RPSA). With this purpose, the kinetics and equilibrium of adsorption of CO2/H2O/N2 mixtures on three honeycomb carbon monoliths (793, 932, and AM03) were assessed in a thermogravimetric analyzer (TGA) under different postcombustion capture scenarios (temperature of 50 °C and several concentrations of CO2). The kinetics study exhibited that the single adsorption of CO2 and H2O can be adequately described by the Avrami and exponential decay-2 models, respectively. As expected, the three carbon monoliths presented fast adsorption of CO2 from a CO2/H2O mixture. Furthermore, when humid flue gas was considered, overall adsorption kinetics were governed by CO2. Besides, the experimental data fitting to the intraparticle diffusion model showed that gradual CO2 and H2O diffusion toward the micropores was the rate-limiting stage. The obtained results give a better insight into the selective adsorption of CO2 and the potential of honeycomb carbon monoliths to separate CO2 from humid flue gas in the context of the cement industry. Carbon monolith 793 is the best carbon monolith candidate to capture CO2 under the evaluated conditions: a capacity of adsorption of 1 mmol of CO2 g–1 and favorable kinetics in 32 vol % CO2 and 4 vol % H2O(v), at 50 °C and 101.3 kPa.


Temperature Programmed Desorption (TPD) tests
Temperature programmed desorption (TPD) tests were carried out in a thermogravimetric analyzer, Setaram TGA92, coupled to an OmnistarTM mass spectrometer from Pfeiffer Vacuum. Around 70 mg of carbon sample were placed in an aluminum crucible (170 µL) and heated from ambient temperature to 1000 °C (heating rate of 15 °C min -1 ) under flowing argon (50 cm 3 min -1 ). Before these measurements, calibration tests with calcium oxalate were carried out. Upon heating in an inert atmosphere, the oxygen surface complexes of carbonaceous materials decompose, releasing CO₂ and CO. CO₂ results from the decomposition of carboxyls, lactones, and anhydrides, while CO comes from anhydrides, phenols, carbonyls, quinones, and pyrones. In the mass spectrometer, the mass to charge (m/z) values 18, 28, and 44 were selected to monitor the evolution of H₂O, CO, and CO₂, respectively. Scientific data analysis and graphing software helped with the fitting of the TPD curves with GaussianAmp peaks. There was no baseline subtraction. Figure S1 shows the TPD plots (CO₂ and CO evolution) for the honeycomb carbon monoliths. Overall, the main peak in the CO₂ profiles appears at around 330 °C and it is associated with less acidic carboxylic groups. The large tail in the CO₂ profile that leads to a peak at ~ 568 °C indicates the presence of peroxides [1] and the second main peak located at 1036 °C is assigned to more stable oxygenated groups, such as lactones [2,3].
Likewise, the main peak of CO desorption takes place at 800 °C and is ascribed to the evolution of carbonyls and quinones; at 937 °C, CO desorption continues due to the decomposition of pyrone and chromene groups, whose contributions are difficult to isolate [2]. Besides, the appearance at lower temperatures of other contributions may be due to the thermal decomposition of carbonyl groups in αsubstituted ketones and aldehydes [3].
On the other hand, desorption of oxygen groups in the form of CO and CO₂ continues above 1000 °C.

CO₂ desorption tests
The CO₂ adsorption capacity was determined from the amount of CO₂ desorbed to show that during the multicomponent tests on the carbon monoliths the CO2 uptake reaches the equilibrium even in the presence of a small concentration of water vapor.
By coupling an Omnistar TM mass spectrometer from Pfeiffer Vacuum to the thermogravimetric analyzer, the mass to charge (m/z) 44, 18, and 28 were monitored to account for the evolution of CO₂, H2O and N2 during the desorption step following the adsorption step in the multicomponent experiments.
The desorption was conducted by heating the sample from 50 °C and 200 °C at a heating rate of 15 °C min -1 at a nitrogen flow rate of 100 mL min -1 .
Herein, the performances of the honeycomb monolith 793 will be shown in the Figures for illustrative purposes. Figure S2(a) and S2(b) show the m/z 44, 18, and 28 signals evolution during the desorption step corresponding to an experiment feeding a ternary gas mixture (32 vol.% CO₂, 4 vol.% H₂O, N₂ balance) at 50 °C and atmospheric pressure on sample 793. Figure S2(a) shows the CO₂ desorption profile wherein a well-defined peak prevails in the first minutes of the desorption step and then the desorption of CO2 slows down and there is a large tail until the end of the experiment when the regeneration is completed. The desorption of H2O is delayed and starts at a later stage. Figure S2(b) shows the N2 profile that tends to a plateau given that it is the sweeping gas in the desorption stage. On the other hand, the TGA profile during the desorption stage ( Figure S3) exhibits a very fast drop in the mass in the first few minutes, due to the rapid desorption of CO₂ (ca. 3.4 wt.% released after ~4 min that corresponds to approximately 62% of the total uptake). Then desorption continues at a slower pace because of the water vapor contribution and reaches a constant mass within ~10 min indicating the full regeneration of the adsorbent (see Figure S3). Although the MS analysis is slightly delayed in time from the TGA, there is a good correspondence between the results in Figures S2 and S3. The above-mentioned confirms that over the first 4 min of the desorption step only CO2 is desorbed. Therefore, the amount of CO2 desorbed over that time frame indicates the extent of the CO2 adsorption. Calcium oxalate calibration was used to quantify CO2.
All the intensities of the peaks were expressed in relative terms to maximum and minimum values and normalized to the maximum intensity (N₂ signal) with the following expression: Making the correspondence between the integrated area (A·s) within the selected time range and the quantities of CO2 from the calcium oxalate calibration we estimated the amount of CO2 desorbed. Figure S4 illustrates the normalized m/z 44 signal that corresponds to the CO₂ desorbed from sample 793. The area below the curve for the first 4 min led to a final number of 1.02 mmol g -1 of CO2 desorbed that equals the CO2 uptake at equilibrium at 50 ºC and a CO2 partial pressure of 32.1 kPa for sample 793.
These results show that the amount of CO2 desorbed matches the CO2 adsorption at equilibrium at the corresponding partial pressure and temperature, and confirm that the CO2 uptake prevails during the adsorption stage when the relative humidity of the feed is low.