Materials like zeolites and metal organic frameworks (MOFs) are microporous in nature, which makes them a potential candidates for catalysts, absorbents, and in separation processes.
For instance, a highly flexible microporous MOF material effectively captures and separates carbon dioxide from other smaller gases (such as nitrogen, hydrogen, methane, carbon monoxide and oxygen) with a separation ratio of 294, 190, 257, 441 and 768 for CO2/N2, CO2/H2, CO2/CH4, CO2/CO and CO2/O2 respectively, at 25°C and 0.16 atm.
Hence, gaining insights into the sorption behavior of microporous materials is of interest for liquid and gas capture and separation. This article discusses the sorption of ethanol/water vapor, SO2 and CO2 in zeolite, SiO2, and Zn-MOF, respectively.
A DVS Vacuum instrument was used to perform all measurements. The Vacuum system with its 400°C pre-heater allows micropores to completely empy, which is otherwise difficult to achieve by thermal heating and dry gas flow alone.
CO2 Sorption in Zn-MOF
Before performing CO2 sorption measurements, outgassing of the Zn-MOF sample supplied with 88.4m2/g BET surface was carried out at 25°C and 10-5 Torr for 3h. This was followed by the introduction of CO2 gas into the system with a partial pressure range of 0-722 Torr and step duration of 20min. The CO2 absorption kinetics data in the MOF sample at 25°C are presented in Figure 1.
Figure 1. Dynamic sorption data of CO2 gas on Zn-MOF material at 25°C
The adsorption and desorption mechanisms are analogous and a total uptake of 0.7462mg is observed. Figure 2 presents the plot of the corresponding isotherm with Type I isotherm characteristics. This sorption behavior is characteristic of that of a microporous material.
Figure 2. Adsorption branch of CO2 gas on Zn-MOF material at 25°C
The isotherm has not plateaued to a constant value, suggesting that more CO2 may be captured at higher partial pressures, or pressures above atmosphere. MOF materials have relatively large surface areas and it is possible to design their pore size by varying preparation conditions for capturing or separating a particular gas.
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SO2 Sorption Studies on SiO2 and Zeolite
The pre-heater was used to dry appromiximately 50mg of microsilica sample at 200°C and under vacuum (10-5 Torr) for 2h in order to achieve equilibrium mass. A temperature-controlled incubator was used to perform the experiment at 25°C. The dry mass (m0) was first established by maintaining the sample at low pressure (10-5 Torr and 0% P/P0).
The adsorbate used for the analysis was pure industrial SO2 gas (purity > 99.98%; CAS No. 7446-09-5). The sample was then subjected to SO2 gas exposure from 0% to 60% P/P0 in 10% steps.
A maximum pressure of 760 Torr was set as the basis for the partial pressures, providing a range of applied pressure of 0-456 Torr. In a similar fashion, the P/P0 was reduced to complete a full adsorption/desorption cycle.
Figure 3 depicts a superimposed plot of the net percentage change in mass (based on dry mass) against time. This reveals the two samples with different densities due to different impurities show similar SO2 gas adsorption/desorption behavior.
The SO2 gas was adsorbed in small quantities by both samples, with significantly fast sorption and desorption kinetics. However, the adsorption capacity of Sample 1, with a higher density, is slightly higher than that of Sample 2 (by merely 4.7% mass at 60%P/P0).
Figure 3. Dynamic sorption data on two different SiO2 samples
Figure 4 presents the corresponding SO2 sorption/desorption isotherm plots collectively, displaying the percentage change in mass (referenced from m0) against the target % P/P0.
A similar isotherm profile can be observed for both samples. However, an open-ended isotherm shape of Sample 2 is indicative an irreversible uptake of SO2 molecules (≤ 0.05%mass).
Figure 4. Isotherm plot of SO2 on SiO2 samples
Figure 5 shows the SO2 isotherm of zeolite 5A activated at 400°C in vacuum before taking the sorption measurement. Since SO2 has a reported molecular diameter of roughly 6Å, therefore zeolites having a pore size comparable to the gas molecule must be more efficient.
This has been demonstrated by the high adsorption capacity in the low partial pressure range (< 10% P/P0), where the pore adsorption potential predominantly influences the pore filling process. This means that zeolite 5A is suitable candidate for SO2 capture.
Figure 5. Isotherm plot of SO2 in zeolite 5A
Ethanol/Water Co-Adsorption on Zeolite 3A
This analysis involved activating a zeolite 3A sample at 10-5 Torr and 400°C for 4h and performing sorption measurements at 130°C using the pre-heater, to ensure that condensation is avoided, which would prevent further vapor adsorption into the micropores.
Figure 6 presents the plot of water sorption isotherm at 130°C, showing 8.29% of water vapor uptake at 20% P/P0 and incomplete filling of the micropores. Water vapor uptake must be higher at higher P/P0.
Figure 6. Water sorption isotherm at 130°C
The ethanol sorption isotherm at 130°C is presented in Figure 7, showing a lower ethanol uptake of 0.90% compared to 8.29% for water at the same P/P0. The ethanol uptake reaches only 1.31% even at 70% P/P0.
The much lower ethanol uptake compared to water vapor is most likely due to the larger molecular size of ethanol (0.44nm compared to 0.28nm of water vapor). Therefore, the zeolite 3A with a pore size of 0.30nm is a potential candidate for the sorption of 0.28nm water molecules.
Figure 7. Ethanol sorption isotherm at 130°C
Gaining insights into co-adsorption behavior is useful for filtration and separation processes. A mixed vapor concentration of 95% ethanol and 5% water was generated by introducing 50sccm ethanol and 2sccm water into the zeolite 3A sample at 130°C. The co-adsorption isotherm for the vapor mixture is shown in Figure 8, where the total uptake of the mixed vapors at 70% P/P0 is 10.13% of the dry mass.
Figure 8. Co-adsorption of 95% ethanol and 5% water
This is in the range of the uptake of water when its isotherm is extrapolated to 70%P/P0, implying that the total sorption greatly depends on water sorption due to its smaller molecular size.
However, there was change in the sorption behavior observed due to competition between the two vapors. A maximum uptake was observed around 40% P/P0 on the adsorption branch due to the absence of ethanol.
There was a gradual decrease in the uptake above 40% P/P0, revealing that surface adsorption, not pore filling, contributes to the higher uptake around 40% P/P0. However, heating to 130°C eliminated this additional surface adsorption.
The significant hysteresis on desorption branch, as indicated by a considerable drop in mass below 20% P/P P0, implies trapping of water in pores by capillary forces or formation of hydration to some extent. These results indicate the applicability of zeolite 3A for capture and separation of water from ethanol.
This article discussed the analysis of zeolite and microporous MOF samples with different pore sizes at different temperatures to measure gas and water/ethanol vapor mixture sorption isotherms.
Gas uptake can be optimized by designing the pore size and surface area of MOF materials. The results revealed that microporous materials are more suitable than SiO2 for SO2 sorption, and the zeolite 3A is a good candidate to capture water molecules of 0.28nm size from water/ethanol mixtures.
This information has been sourced, reviewed and adapted from materials provided by Surface Measurement Systems Ltd.
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