Sustainable synthesis of Fe-MOR zeolite for efficient capture of CO2
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Abstract
Selective adsorption of carbon dioxide (CO2) is significant for carbon neutrality, where searching for efficient CO2 adsorbents is very important. In addition, coal fly ash (CFA) is one of the largest industrial solid wastes with environmental damages, where conversion of the wastes into costly functional materials is attractive. This work showed sustainable synthesis of Fe-containing mordenite (Fe-MOR) zeolite from the CFA waste under solvent-free conditions, and this zeolite is an efficient capturer for CO2 in the mixture of CO2/N2 (15/85, v/v), giving adsorption capacity of 2.07 mmol/g and separation coefficient of 58.9 at 298 K. Very interestingly, the capture of CO2 in the mixture of CO2/N2 (15/85, v/v) is recyclable. This work not only solved the accumulation and pollution of CFA but also prepared a highly efficient adsorbent of Fe-MOR zeolite, which would open a door for utilizing environmentally unfriendly solid wastes as value-added functional materials in the future.
Keywords
INTRODUCTION
The rapid increase of greenhouse gases, particularly carbon dioxide (CO2), is one of the global problems that could increase the likelihood and severity of natural disasters such as wildfires, heatwaves, and droughts[1-8]. Since renewable energy resources are still being developed and thus fossil fuels will remain the dominant energy source for a long time, it is crucial to efficiently curb CO2 emissions[9-17]. At present, great efforts have been made in developing efficient technologies for capturing CO2[18-22]. Among them, it is generally believed that solid porous adsorbents with low cost, high capacity and selectivity of CO2 are critical.
Zeolites, as a class of typically solid porous adsorbents with ordered micropores, are widely employed in fields such as catalysis, ion exchange, adsorption, and separation due to their high surface areas, large micropore volumes, and good stabilities[23-26]. In recent years, capturing CO2 by zeolites has become a hot topic due to their excellent stability for zeolite frameworks[3,4,18,27-32]. For example, Zhou et al. first reported that Fe-containing mordenite (Fe-MOR) zeolite synthesized from the “acid co-hydrolysis route” had narrowed microchannels, which was efficient for CO2 adsorption and separation[33]. However, this method, which uses tetraethyl orthosilicate (TEOS) as a starting raw material and generates wastewater in the hydrothermal synthesis, incurs relatively high costs. In addition, conventional synthesis of Fe-MOR zeolite failed to obtain these narrowed microchannels under hydrothermal conditions due to the formation of aggregated Fe species. Thus, it is highly desirable to develop a simple and sustainable route for synthesizing Fe-MOR zeolite with high capacity and selectivity of CO2.
Recently, we developed a simple and sustainable route for solvent-free synthesis of zeolites, which completely avoided using water solvents[34-38], where the aggregation of Fe species was effectively hindered[39]. Therefore, it offers the possibility of preparing Fe-containing zeolites without aggregation of Fe species under solvent-free conditions. Considering that coal fly ash (CFA), mainly containing silica, alumina, and iron oxides, one of the largest industrial solid wastes emitted from the combustion of coal powder, is environmentally unfriendly[40-45], it is really sustainable for the conversion of the CFA waste into highly efficient adsorbent of Fe-MOR zeolite under solvent-free conditions.
This work, for the first time, showed a simple and sustainable synthesis of Fe-MOR zeolite from CFA waste under solvent-free conditions. As expected, the Fe-MOR zeolite exhibited good adsorption capacity for CO2 (2.07 mmol/g at 298 K), high separation coefficient (58.9 at 298 K), and excellent recyclability. These features, plus sustainable synthesis, are helpful for potential applications of this zeolite in the future.
EXPERIMENTAL
Solvent-free synthesis of Fe-MOR zeolite from CFA
Experimental sections, including materials and characterizations [Supplementary Materials].
For the solvent-free synthesis of Fe-MOR zeolite from CFA (Fe-MOR-CFA), the CFA and NaOH powder were mixed at a mass ratio of 1:1.2, calcined at 800 °C for 2 h, cooled to room temperature, and ground into a fine powder at room temperature[37]. The chemical composition of the CFA and the activated CFA by alkali fusion were measured with X-ray fluorescence (XRF) [Supplementary Table 1].
As a typical run for the solvent-free synthesis of Fe-MOR zeolite, 0.534 g of CFA fine powder, 0.879 g of solid silica, and 0.1 g of MOR zeolite as seeds were ground for 10 min. Subsequently, the obtained power was poured into an autoclave, and 1.2 g of H2O was added. The product was finally crystallized at 175 °C for 10 h and designated as Fe-MOR-CFA. The yield of the Fe-MOR-CFA zeolite is approximately 90%. The Si/Al ratio and Fe content of the Fe-MOR-CFA zeolite were 6.9 and 0.77 wt.% by inductively coupled plasma optical emission spectroscopy (ICP-OES) measurements, which was consistent with the result of XRF
For comparison, Fe-MOR zeolite was synthesized from an “acidic co-hydrolysis route” accordingly[33], which was designated as Fe-MOR-ACH. The Si/Al ratio and Fe content of the Fe-MOR-ACH zeolite were 6.5 and 0.73 wt.% by ICP-OES measurements.
Gas adsorption
Adsorption data for CO2 and N2 at 273 and 298 K were carried out in the Physical Adsorption Analyzer [Beishide Instrument Technology (Beijing) Co., Ltd., China]. Adsorption selectivity was based on ideal adsorbed solution theory (IAST)[33] for CO2/N2 (15/85, v/v), determined by:
Where x and y were the mole fraction of absorbate in the adsorption and gas phases, respectively. The subscripts i and j were on behalf of CO2 and N2.
Isosteric heat of adsorption[46] was calculated from the Clausius-Clapeyron equation to predict the interactions between the adsorbate and the adsorbent, using the CO2 adsorption-desorption at 273 and
Where P was the pressure, R was the ideal gas constant, 8.314 kJ/mol, C was the constant, T was the temperature, K, and ∆Hs was calculated from slopes of plots of In Pvs. 1/T of fixed loading.
The recyclability of single-component CO2 adsorption was obtained on the Weight Method Vacuum Steam Absorption [Beishide Instrument Technology (Beijing) Co., Ltd., China].
CO2 dynamic adsorption profiles for sole composition at 298 K were measured on the Gravity Vacuum Vapor Sorption Test Report [Beishide Instrument Technology (Beijing) Co., Ltd., China]. Firstly, 0.06~
The CO2/N2 breakthrough separations were performed in a multi-component adsorption breakthrough curve analyzer [Beishide Instrument Technology (Beijing) Co., Ltd., China] at 298 K. Firstly, 0.4~1.0 g of as-synthesized powder was placed in the quartz tubing (6 mm diameter) and then purged with He flow (5 mL/min) for 2 h at 573 K. The mixture (CO2/N2: 15/85, v/v) flows without water condition, with water condition and the presence of SO2 (1,000 ppm) were then introduced at 3.0 mL/min at 298 K. The flow rates of all gas mixtures were regulated by mass flow controllers; the humidity was controlled and detected using a multi-component adsorption breakthrough curve analyzer, and the CO2 and N2 concentrations from the adsorbent bed were continuously monitored with a gas analytical mass spectrometer. Besides, the zeolite was regenerated in situ in the column with a 5 mL/min flow at 573 K for 2 h.
RESULTS AND DISCUSSION
Figure 1 displayed the schematic representation of solvent-free synthesis of Fe-MOR-CFA zeolite. Figure 2 showed the X-ray diffraction (XRD) pattern, scanning electron microscopy (SEM) image, N2 sorption isotherms, and 27Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectrum of Fe-MOR-CFA zeolite. The XRD pattern of the Fe-MOR-CFA zeolite [Figure 2A] displayed peaks with good resolution in the range of 5°-40°, which are well consistent with those of the simulated MOR structure
Figure 1. Schematic representation of solvent-free synthesis of Fe-MOR-CFA zeolite from coal fly ash. Fe-MOR-CFA: Fe-containing mordenite zeolite from coal fly ash.
Figure 2. (A) XRD pattern; (B) SEM image; (C) N2 sorption isotherms; and (D) 27Al MAS NMR spectrum of the Fe-MOR-CFA zeolite. XRD: X-ray diffraction; SEM: scanning electron microscopy; MAS: 27Al magic-angle spinning; NMR: nuclear magnetic resonance; Fe-MOR-CFA: Fe-containing mordenite zeolite from coal fly ash.
Ultraviolet-visible (UV-vis) and electron spin resonance (ESR) techniques were used to investigate the chemical state of Fe species. UV-vis spectra of the Fe-MOR-CFA and Fe-MOR-ACH zeolites displayed two monomer bands centered at 210-220 nm and 250-265 nm, respectively [Figure 3A]. The 210-220 nm band was assigned to tetrahedrally coordinated Fe3+ ions, while the 250-265 nm band was attributed to isolated Fe3+ ions with higher coordination[47,48]. Notably, no peaks between 300 and 400 nm in the Fe-MOR-CFA and Fe-MOR-ACH zeolites were associated with oligonuclear FexOy clusters[49]. Their ESR spectra [Figure 3B] exhibited the signal at g = 4.3 associated with isolated Fe3+ ions with tetrahedral coordination and the signal at g = 2.0 related to Fe species inside microporous channels[33,50]. The above results revealed that the aggregated Fe species were excluded in these zeolites, confirming the feasibility of solvent-free synthesis of Fe-MOR-CFA zeolite from CFA.
Figure 3. (A) UV-vis and (B) ESR spectra of Fe-MOR-CFA and Fe-MOR-ACH zeolites. UV-vis: Ultraviolet-visible; ESR: electron spin resonance; Fe-MOR-CFA: Fe-containing mordenite zeolite from coal fly ash; Fe-MOR-ACH: Fe-containing mordenite from acidic co-hydrolysis route.
Supplementary Figures 6 and 7 exhibited the XRD patterns and SEM images of the Fe-MOR-CFA zeolite samples with different ratios of the MOR zeolite seeds to SiO2 in the mixture of initial raw materials from 0 to 0.20. Notably, when no MOR zeolite seeds were added (a ratio of 0), the crystal size of the Fe-MOR-CFA zeolite was very large (with a diameter larger than 3 μm). However, when the zeolite seeds were included, the crystal sizes significantly decreased (with a diameter of less than 0.5 μm). These results suggested that a suitable ratio of the MOR zeolite seeds to SiO2 is significant for adjusting zeolite crystal sizes.
Supplementary Figure 8 showed the effect of the Fe-MOR-CFA zeolite on the Na2O/SiO2 ratios in the mixture of initial raw materials. Pure Fe-MOR-CFA zeolite could be obtained when the ratios of Na2O/SiO2 were adjusted from 0.22 to 0.26 [Supplementary Figure 8A and B]. When the ratios were higher than 0.29, the ANA phase was observed in the zeolite product [Supplementary Figure 8C]. Therefore, the Na2O/SiO2 ratios should be seriously regulated in the solvent-free synthesis of the Fe-MOR-CFA zeolite.
Figure 4 showed the crystallization process of the Fe-MOR-CFA zeolite explored by XRD and SEM techniques. Firstly, very weak peaks of MOR zeolite before crystallization were observed, which was related to the MOR zeolite seeds in the mixture of initial raw materials [Figure 4A(a)]. A series of peaks associated with MOR structure appeared when the crystallization time was 4 h [Figure 4A(b)]. Meanwhile, a few Fe-MOR-CFA zeolite crystals could be observed obviously from the SEM image [Figure 4B(b)], which was extremely consistent with the XRD pattern. The zeolite crystals formed quickly from 4 to 6 h, as observed from both XRD patterns and SEM images [Figure 4A(c) and B(c)]. The XRD intensity of the obtained samples did not change after the crystallization for 10 h, meaning a full crystallization [Figure 4A(e)]. Accordingly, more Fe-MOR-CFA zeolite crystals could be attained [Figure 4B(e)]. The dependence of the Fe-MOR-CFA zeolite crystallinity on crystallization time was shown in Supplementary Figure 9.
Figure 4. (A) XRD patterns and (B) SEM images of the Fe-MOR-CFA zeolite crystallized at (a) 0, (b) 4, (c) 6, (d) 8, (e) 10, and (f) 12 h, respectively. XRD: X-ray diffraction; SEM: scanning electron microscopy; Fe-MOR-CFA: Fe-containing mordenite zeolite from coal fly ash.
Quantitative porosity was analyzed using N2 and Ar sorption experiments. Both the Fe-MOR-CFA and Fe-MOR-ACH zeolites exhibited extremely low N2 and Ar uptake [Figure 5, Supplementary Figures 10 and 11, Supplementary Table 2], suggesting that both with narrowed orifices retard larger N2 (3.64 Å) and even Ar (3.4 Å)[33]. In contrast, both samples enable CO2 to enter with a small kinetic diameter (3.3 Å), as supported by CO2 sorption isotherms of the samples [Figure 5]. The CO2 adsorption capacities of the Fe-MOR-CFA zeolite at 273 and 298 K were 3.31 mmol/g and 2.93 mmol/g, respectively, which is comparable with those (3.45 and 3.09 mmol/g) of the Fe-MOR-ACH zeolite.
Figure 5. CO2 and N2 sorption isotherms of (A) Fe-MOR-CFA zeolite and (B) Fe-MOR-ACH zeolite at 273 and 298 K. Fe-MOR-CFA: Fe-containing mordenite zeolite from coal fly ash; Fe-MOR-ACH: Fe-containing mordenite from acidic co-hydrolysis route.
Figure 6A showed adsorption selectivity of CO2/N2 (15/85, v/v) calculated by single-component CO2 and N2 sorption isotherms at 273 and 298 K for Fe-MOR-CFA and Fe-MOR-ACH zeolites, giving excellent separation ability. Isosteric heats of CO2 adsorption (Qst) as a function of CO2 uptakes were analyzed [Figure 6B], giving similar values for these two zeolites. In addition, they have similar CO2 adsorption kinetics [Supplementary Figures 12 and 13], meaning similar electrostatic interaction between the iron species and CO2[33,51]. After recycling ten times, no loss for CO2 uptake at 298 K was observed for the Fe-MOR-CFA zeolite [Figure 6C], indicating its recyclability for adsorption and desorption of CO2. Furthermore, the separation efficiency of the two zeolites was evaluated by the column breakthrough separations for the mixture of CO2/N2 (15/85, v/v) at 298 K and atmospheric pressure. As shown in Figure 6D, N2 breakthrough occurred immediately, while the CO2 retaining time of Fe-MOR-CFA and Fe-MOR-ACH zeolites were 100.4 and 97.0 min/g, respectively, and their CO2 adsorption capacities were 2.07 and 2.02 mmol/g, respectively. From the breakthrough curve, the separation coefficient for Fe-MOR-CFA and Fe-MOR-ACH zeolites were calculated at 58.9 and 53.5 for CO2/N2 (15/85, v/v), respectively, where the Fe-MOR-CFA zeolite exhibited the separation coefficient even better than Fe-MOR-ACH zeolite, one of the best zeolite adsorbents of CO2 yet. The better performance might be associated with K+ and Ca2+ in the raw materials (CFA)[52]. In addition, the column breakthrough separations of the Fe-MOR-CFA zeolite with a relative humidity (RH) of 60% and the presence of SO2 (1,000 ppm) were performed at 298 K
Figure 6. (A) IAST prediction of CO2/N2 (15/85, v/v) selectivities on Fe-MOR-CFA zeolite and Fe-MOR-ACH zeolite at 273 and 298 K; (B) Isosteric heats (Qst) as a function of CO2 uptakes on Fe-MOR-CFA zeolite and Fe-MOR-ACH zeolite; (C) CO2 adsorption isotherms of Fe-MOR-CFA zeolite for ten times at 298 K; (D) Experimental column breakthrough curves for CO2/N2 (15/85, v/v) separations on Fe-MOR-CFA zeolite and Fe-MOR-ACH zeolite at 298 K. IAST: Ideal adsorbed solution theory; Fe-MOR-CFA: Fe-containing mordenite zeolite from coal fly ash; Fe-MOR-ACH: Fe-containing mordenite from acidic co-hydrolysis route.
Finally, the solvent-free synthesis of the Fe-MOR-CFA zeolite was scaled up to 100-200 mL autoclaves
CONCLUSIONS
In summary, it is successful for solvent-free synthesis of Fe-MOR-CFA zeolite with high yields using solid waste of CFA as aluminum, silicon, and iron sources. This route not only consumes solid waste of CFA but also produces value-added CO2 adsorbents with high adsorption capacity, excellent separation coefficient, and good recyclability. The sustainable synthesis plus excellent features for capturing CO2 could provide a good opportunity for potential applications of the Fe-MOR-CFA zeolite in the future.
DECLARATIONS
Acknowledgments
We sincerely thank all the team members who participated in this study.
Authors’ contributions
Zeolite preparation, characterization, and the draft manuscript: Liu P
Discussion on the results: Yan K, Wang L
Design of the study, data analysis, and manuscript writing: Wu Q, Xiao FS
Availability of data and materials
Synthetic procedure and results of characterizations can be found in the Supplementary Materials. The data that support the findings of this study are available from the corresponding author upon reasonable request.
Financial support and sponsorship
This work was supported by the National Key Research and Development Program of China (2022YFA1503602), Shanxi-Zheda Institute of Advanced Materials and Chemical Engineering (2021SZ-AT003), and the National Natural Science Foundation of China (22288101 and 22172141).
Conflicts of interest
Wang L is the Junior Editorial Board Member of Chemical Synthesis. Both Wang L and Xiao FS are the guest editors of the Special Issue of “Zeolite for Sustainable Catalysis”, while the other authors have declared that they have no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2024.
Supplementary Materials
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