study

1. Evaluation of porous adsorbents for CO2 capture
under humid conditions: The importance of
recyclability
As for 1°-MCM-41, its poor adsorption kinetics made the CO2 adsorption under dynamic
conditions to be far lower than the adsorption at equilibrium
Chong Yang Chuah, Wen Li, Yanqin Yang
2020

2. Synopsis
Researchers have used smartphones to investigate the relationship between pain and weather... and their findings.
The study app was downloaded by 13,207 users over the 12-month recruitment period.
A total of 10,584 participants had complete baseline information and at least one pain entry.
The ‘worst’ combination of weather variables would increase the odds of a pain event by just over 20% compared to an average
day.
Higher relative humidity and wind speed, and lower atmospheric pressure, were associated with increased pain severity in people
with long-term pain conditions.
The effect of weather on pain was not fully explained by its day-to-day effect on mood or physical activity.
The analysis involved 2658 patients.
However, “The study was advertised to participants with a clear research question, authors say.
Rain and cold weather were the most common pre-existing beliefs, they say.

3. Key concepts
• Fourier-transform infrared spectroscopy
• HKUST-1
• synthesis
• equilibrium
• metal-organic frameworks
• Recyclability
• molecular dynamics
• Water vapor
• carbon dioxide
• MCM-41
• carbon capture
• density functional theory
• zirconium
• kinetics
• carbon capture and sequestration

4. Abstract
Breakthrough measurement has been the most common method for studying multicomponent adsorption of porous materials with a
feed gas containing CO2, N2 and H2O. However, most studies of breakthrough measurement on novel adsorbents have focused on
the CO2 adsorption that occurs during the first breakthrough run, thus leading to the misleading conclusion that the synthesized
porous adsorbents are capable of capturing very large volumes of CO2 even in the presence of water vapor. Therefore, we
conducted evaluations of CO2 adsorption on porous adsorbents by breakthrough measurement under dynamic conditions, using a
range of commonly reported porous adsorbents and focusing on recyclability and kinetics. Aside from during a first run with fresh
samples, most adsorbents showed poor recyclability in humid conditions. However, HKUST-1 and 1°-MCM-41 could capture CO2
under humid conditions over successive adsorption–desorption cycles. We also found that the sorption kinetics could be important
for optimal utilization of amine-appended sorbents in industrial carboncapture processes.

5. Confirms previous work
For instance, the hydrothermal stability of porous materials has been rigorously evaluated by subjecting the samples under
repetitive adsorption–desorption cycling process [20]. Thus, in this study, the performance of three different types of porous
adsorbents (zeolite and zeotype frameworks, MOFs and amine-impregnated silicas) were evaluated under repetitive adsorption–
desorption cycling to demonstrate the importance of such recyclability for practical application of solid adsorbents in post-
combustion carbon-capture process

6. Counterpoint to earlier claims
It is hypothesized that a small loading of water molecules in HKUST-1 increases the Coulombic interaction between water and
CO2 molecules, resulting in the similar CO2 adsorption capacity of the material in these humid conditions as under dry conditions.
Such an effect is not observed in Ni2dobdc despite its possessing open metal sites, like HKUST-1 [32,59,60,61]

7. Builds on previous research
On the other hand, the details of the experimental designs for adsorption–desorption cycling are given below. The investigation of
gas adsorption performance of adsorbents under dynamic conditions was performed based on the custom-built breakthrough system
developed in our previous work [32] (Fig. S1)
S3–8; Table S1-2). In general, the properties of all adsorbents used in this study are comparable to those of other adsorbents
reported in the literatures [23,26,29,31]

8. Differs from previous work
In general, conclusions based on the observation of the breakthrough measurement during only one cycle under a supply of humid
test gas [23] can be considered insufficient. Breakthrough measurement is considered as the most direct and relevant way to
explore such performance, but under humid conditions such measurements can provide misleading results although the analysis is
correctly performed.

9. Highlights
• The deployment of carbon capture and sequestration (CCS) may be necessary, as it enables CO2 to be adsorbed from the point
source, as well as the transportation and storage of CO2 deep underground to prevent its release to the atmosphere
• Prior to the adsorption study, all adsorbents synthesized were characterized by N2 physisorption at 77 K, TGA, Fourier-
transform infrared spectroscopy (FT-IR) and elemental analyses, and all the data from these analyses are presented in the
Supplementary Information
• The results clearly showed that most adsorbents had reasonably stable CO2 adsorption capacity throughout multiple
adsorption–desorption cycles with a dry feed gas (Fig. 6a–g)
• The adsorption capability of the porous adsorbents examined in this study was generally poor in the presence of water during
repetitive adsorption–desorption cycling, aside from that of HKUST-1 and 1°-MCM-41, which retained the ability to adsorb
CO2 under humid conditions
• As for 1°-MCM-41, its poor adsorption kinetics made the CO2 adsorption under dynamic conditions to be far lower than the
adsorption at equilibrium
• It is important that novel porous adsorbents are subjected to several (≥10) adsorption–desorption cycles to ensure that their
CO2-adsorption capability can be recovered under mild regeneration conditions

10. Introduction
Since the onset of the industrial revolution in the 19th century, the concentration of CO2 in the atmosphere has been steadily
increasing, and surpassed 400 ppm in 2015 [1,2,3].
It is important that an alternative mode of carbon capture is developed to reduce the overall cost of this process [12,13,14–15]
Porous adsorbents such as zeolites, metal-organic frameworks (MOFs) and mesoporous silica have attracted attention as
alternatives to amine solutions [16,17,18–19].
Studies on multicomponent adsorption of CO2/N2/H2O are limited, and measurement of the CO2-capture capability of adsorbents
under repetitive adsorption–desorption cycling under dynamic condition is still rarely reported, especially in humid conditions
Such measurements are critical for the rigorous evaluation of adsorbents for practical applications.
In this study, the performance of three different types of porous adsorbents were evaluated under repetitive adsorption–desorption
cycling to demonstrate the importance of such recyclability for practical application of solid adsorbents in post-combustion carbon-
capture process.

11. Materials
2,5-dihydroxy-1,4-benzenedicarboxylic acid (H4dobdc; Tee Hai Chemical), (3-aminopropyl)trimethoxysilane (APTMS; Sigma
Aldrich), copper(II) nitrate trihydrate (Cu(NO3)2.3H2O; Sigma Aldrich); mesoporous silica (MCM-41; Sigma Aldrich); nickel (II)
nitrate hexahydrate (Ni(NO3)2.6H2O; Sigma Aldrich); potassium fluoride (KF; VWR), sodium hydroxide solution (NaOH; Sigma
Aldrich); sodium silicate solution (Na2SiO3; 17–19% Na2O and 35–38% SiO2; Kanto Kagaku); sulfuric acid (H2SO4; Sigma
Aldrich), titanium (IV) isopropox-.
The following solvents were used as received: absolute ethanol (VWR), deionized water (H2O), hexane (VWR) methanol (VWR),
N,N’dimethylformamide (DMF; VWR), toluene (VWR) and triethylamine, (TEA; Sigma Aldrich) were used as received without
further purifications

12. Synthesis of adsorbents
Zeolite 5A and Zeolite 13X: These were purchased from Sigma-Aldrich and used as received.
5.7 g of Ti[OCH(CH3)2]4 and 1.5 g of H2SO4 were combined in 35 ml of H2O, and the resulting mixture was heated at 100°C for
1.5 h, and cooled to room temperature
This cooled “Ti source solution” was added dropwise to the “Si source solution” with stirring, and the resulting “Si and Ti source
solution” was stirred for additional 1 h.
To ensure that the resulting solid was free of impurities, it was re-dispersed in DMF and heated at 100 °C for 2 h
Such washing-centrifugation was repeated with methanol at room temperature.
The resulting suspension was cooled to room temperature, and the solid was isolated by vacuum filtration, and washed with
copious amounts of toluene.
The resulting samples were dried by heating under vacuum at 60 °C for 24 h

13. Characterizations
The CO2-adsorption properties of adsorbents at 25 °C at 0–1 bar were measured by volumetric gas sorption analyzer using pure
CO2 (Airliquide).
The adsorption kinetics of CO2 at 25 °C and 1 bar dosing pressure were obtained by analysis according to the literature procedure
[29].
The porosity properties of adsorbents were quantified by N2 physisorption measurements at 77 K, after activation of adsorbents as
per the above-described method.
The mesopore size-distribution of adsorbents was determined via the Barret–Joyer–Halenda (BJH) method, based on the adsorption
data.
The thermal properties of adsorbents were determined using a thermogravimetric/differential thermal analyzer (PerkinElmer,
Diamond TG/DTA) in the 25–800 °C range.
Additional analyses of 1°-MCM-41 were conducted by Fourier-transform infrared spectroscopy (FT-IR) at 4000– 500 cm−1 with a
resolution of 4 cm−1 (PerkinElmer, Spectrum One), and by CHNS Elemental Analyzer (Elementar), to determine the C, H and N
composition of the sample

14. Gas adsorptions under dynamic flow condition
The investigation of gas adsorption performance of adsorbents under dynamic conditions was performed based on the custom-built
breakthrough system developed in our previous work [32] (Fig. S1).
The powdered sample to be tested was placed inside the adsorption cell, and the ends were plugged with a small amount of glass
wool.
The samples were activated via flow-degassing, using a continuous flow of argon gas at a designated degassing temperature [32].
The adsorption cell was cooled to room temperature (25 °C) prior to the start of the breakthrough measurement.
The test gas (CO2/N2 at 20/80 vol%, Airliquide) was propagated through the adsorption cell via a four-way valve.
The flow rates of argon and test gas were fixed at 10 sccm, with the aid of mass flow controller (Cole Parmer, 00278SH).
The composition of the test gas at the outlet of the adsorption cell was detected by mass spectrometer.
Throughout the measurement, the pressure inside the adsorption cell was maintained constant at 1 bar.
The details of the experimental designs for adsorption–desorption cycling are given below

15. Adsorption–desorption cycling
The performance of adsorbents under the feed stream containing CO2/N2/H2O were verified by 11 cycles of repetitive adsorption–
desorption cycling.
Given that conducting desorption under very rigorous conditions would be cost-prohibitive in real industrial settings, desorption
conditions for all adsorbents were fixed at 100 °C for 20 min.
After the degassing process and once the temperature of the adsorption cell was settled at room temperature, breakthrough analysis
was conducted by passing dry test gas through the column to analyze the CO2-adsorption performance of the adsorbent.
This process was repeated for 11 cycles

16. Properties of porous adsorbents synthesized
Prior to the adsorption study, all adsorbents synthesized were characterized by N2 physisorption at 77 K, TGA, FT-IR and
elemental analyses, and all the data from these analyses are presented in the Supplementary Information
The properties of all adsorbents used in this study are comparable to those of other adsorbents reported in the literatures
[23,26,29,31].
Using these samples, we have tested CO2-capture performance of three kinds of adsorbents under various conditions

17. Pure CO2 adsorption at equilibrium
CO2 adsorption behavior of porous adsorbents was measured at 25 °C in the pressure range of 0–1 bar using a volumetric gas
adsorption analyzer.
The CO2 adsorption and CO2 adsorption kinetics of the metal-organic frameworks (Ni2dobdc, HKUST-1) at 25 °C and 1 bar
dosing pressure were investigated (Fig. 3a).
These materials were shown by TGA measurement to have reasonable thermal stability up to 300 °C (Fig. S6).
The incorporation of amine moieties reduced the accessible pores in the framework (Fig. S3f), which was evident from the
adsorption kinetics, with saturation remaining incomplete after 100 min

18. Multicomponent adsorption under dynamic flow condition
Evaluating the separation performance of a multicomponent gas mixture under dynamic conditions is typically conducted via
breakthrough measurement
This can be done by passing the test gas mixture to a system where the outlet gas composition can be monitored as a function of
time by mass spectrometry or gas chromatography [23,42,43,44,45,46].
The CO2 adsorption for the first cycle was normalized based on the CO2 breakthrough time of the respective samples in dry
conditions (Fig. 5).
It can be clearly seen from this figure that most of the adsorbents efficiently captured.
Polarizability and quadrupole moment of CO2, N2 and H2O [43 –47]

19. Behavior of adsorbents under the repetitive adsorption–
desorption cycling
The evaluation of CO2 adsorption under multiple adsorption– desorption cycling is important as it estimates the suitability of
adsorbents for long-term operation.
It is reported that the complete desorption of H2O under certain regeneration conditions is often difficult due to the formation of
stable hydrogen bonding network between H2O and the active sites [52,53,54], leading to a gradual decrease in CO2 adsorption
capacity in repetitive adsorption–desorption cycling under humid conditions.
HKUST-1 demonstrated different CO2 adsorption behavior, during multiple adsorption–desorption cycling in humid conditions, in
which its CO2-capture performance was comparable to that in dry conditions
Such behavior has been reported in several studies on the CO2 adsorption of HKUST-1 in the presence of water molecules [57,58].
Additional characterizations (PXRD and N2 sorption at 77 K) of HKUST-1 were conducted to ensure that the crystallinity and
porosity remained intact (Fig. S15 and Table S3)

20. Remarks on sorption kinetics
Adsorption kinetics can be related to the overall processing rate of CO2 adsorption in the industrial process[29].
It is true that the CO2 adsorption under dynamic conditions will be substantially lower than that under equilibrium conditions, as it
is possible in the latter case that CO2 adsorption is yet to be fully equilibrated.
Amine-impregnated adsorbents were unable to achieve such attractive performance, which was possibly due to steric hindrance of
the amine groups restricting effective diffusion of CO2 into the framework.
When designing amine-appended sorbents, one must consider the sorption kinetics as a critical factor affecting their practical utility
in real operations

21. Conclusion
The adsorption capability of the porous adsorbents examined in this study was generally poor in the presence of water during
repetitive adsorption–desorption cycling, aside from that of HKUST-1 and 1°-MCM-41, which retained the ability to adsorb CO2
under humid conditions.
As for 1°-MCM-41, its poor adsorption kinetics made the CO2 adsorption under dynamic conditions to be far lower than the
adsorption at equilibrium
Given these results, it is important that novel porous adsorbents are subjected to several (≥10) adsorption–desorption cycles to
ensure that their CO2-adsorption capability can be recovered under mild regeneration conditions.
The measurement CO2 breakthrough under humid conditions cannot be judged solely on the results of first adsorption–desorption
cycle.
Porous adsorbents should demonstrate reasonably good CO2 adsorption at low partial pressures, high adsorption kinetics, and
reasonable recyclability under repetitive adsorption–desorption cycling, in humid feed-gas conditions.
Future efforts should examine the adsorption–desorption cycling performance of adsorbents in pelletized form, as powdered
samples are generally unsuitable for use in industrial operations due to the large pressure drop and ineffective heat transfer they
exhibit in adsorption cells [62,63]

22. Study subjects
135 papers
Nevertheless, based on the available literature studies on CO2 breakthrough measurements (135 papers from 2015 to 2018, the
reference lists are tabulated and summarized in Supplementary Information in Table S4), only approximately 30% of the
breakthrough measurements of adsorbents’ performance have been conducted under multiple adsorption–desorption cycling in dry
conditions