Introduction

Climate change is a critical world pressing challenge. Emission of greenhouse gases, including nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2), have increased global temperatures by around 1 °C since pre-industrial times, where CO2 alone is contributing to more than 60% of this global warming1,2,3. The increasing percentage of CO2 and CH4 gases into the atmosphere is considered nowadays as a serious threat as it has a range of potential ecological, physical and health impacts, including extreme weather events (such as floods, droughts, storms), sea-level rise, altered crop growth, and disrupted water systems. To mitigate climate change, UN member parties have set a target, in the Paris Agreement, of limiting average warming to 2 °C above pre-industrial temperatures.

From technical viewpoint, the absorption and adsorption of greenhouse gases is one of the most challenging technological issues for researchers and environmentalist towards developing sustainable technologies based on new materials to control and mitigate the anthropogenic emission1. For more than six decades, solvent absorption technologies, and particularly aqueous alkanolamines, were the most commonly employed chemical absorbents for CO2 gas removal.

However, these adsorption processes are associated with various industrial limitations1,4. In the recent development in the chemical sciences field, ionic liquids (ILs) have shown some potential features for CO2 capturing due to their unique chemical and physical properties and replacement capability of serious conventional solvents. In addition, deep eutectic solvents (DESs) have demonstrated many unusual characteristics for CO2 absorption at room temperature5,6,7,8,9, including negligible vapor pressure, non-flammability, wide liquid range, high thermal and chemical stabilities and high solvation capacity10,11. DES term was introduced by the team of Abbott9 in 2003 in order to overcome some of the IL’s drawbacks. Similarly, dried solid materials are another type rational choice for gas adsorption in separation technologies which helps to minimize the energy consumption and regeneration processes up to some extent. Thus, a number of newer solid materials are now being synthesized and characterized and tuned further in order to enhance their practical applicability for specific gas or gas mixtures12,13,14,15. Indeed, gas adsorptivity capacity is closely related to accessible surface structure and molecular arrangement of the solid crystal. Typically, the flue gas composition consists of CO2, H2O, CO, N2, H2S, H2, Ar, NOx, SOx, Hg and Cd16. In some cases, CH4 slip into flue gas stream could occur when the incomplete exothermic oxidation of a fuel took place due to insufficient amount of oxygen to the combustion system. R&D tends to focus on safe and eco-friendly porous organic and inorganic materials for their use in gas storage17,18,19,20. Since then, several inorganic phosphate-based compounds have been extensively studied and were found to not only be able to capture CO2 and CH4 but also to act as storage for clean energy. As a contribution to this internationally critical research effort, we successfully designed and synthesized efficient diphosphates, namely dirubidium cobalt bis(dihydrogendiphosphate) dihydrate (Rb2Co(H2P2O7)2·2H2O), and assessed it as CO2 and CH4 gases adsorption and storage. Our results show a rate of CO2 and CH4 capturing of 3.10 mmol/g and 2.35 mmol/g at temperature of 25 °C and pressure of 50 bar. Moreover, gas sorption comparison study demonstrated a clear evidence of a selective behavior of our compound for CO2 and CH4 gases at each temperature and pressure, and revealed that adsorbtivity of CO2 is more than that of the CH4 at different temperatures and pressures, making this diphosphate a promising candidate for greenhouse gases adsorption and storage. First principles density functional theory (DFT) calculations also reveal strong adsorption of both CO2 and CH4 molecules on the surface of the Rb2Co(H2P2O7)2·2H2O material. The adsorption energy of CO2 molecules is smaller (i.e., stronger adsorption) as compared to the adsorption energy of CH4 molecule due to covalent character of bonding of the CO2 molecule on the surface of the material as revealed in our electronic structure calculations.

The present work focuses on designing a diphosphate that can be used for direct air capture of CO2 and CH4 molecules. This work could lead to promising technology to create CO2 and CH4 outlet from the earth’s atmosphere due to several advantages such as the easy application and geographical independence. The present new configuration can also help bypassing the high cost of DAC systems for greenhouse gas capture.

Methods

Sample fabrication

Single crystals of Rb2Co(H2P2O7)2·2H2O was prepared according to the method reported in our previous works21,22,23,24,25,26. K4P2O7 (1 mmol), Rb2CO3 (1 mmol), and CoCl2.6H2O (0.5 mmol), dissolved in a few ml of diluted HCl (0.75 mL of HCl 0.1 M), as structure directing agent. The mixture was stirred for 2 h. The evaporation of the solvent water was allowed for one week, at room temperature. Prismatic crystals with edge-length up to 0.2 mm was deposited at the end of this period, then filtered-off and washed with water–ethanol mixture (concentration 20:80).

Characterization

The reported crystallographic data for Rb2Co(H2P2O7)2·2H2O are given in Table 1. The Rb2Co(H2P2O7)2·2H2O selected crystal shows an excellent structural quality with exceptional low Rint factors. The hydrogen atoms are localized in different Fourier maps. They are also refined independently using solely the condition that the temperature parameters of hydrogen atoms in H2O molecules and H2P2O7 groups are equal. X-ray diffraction data were collected on an Oxford Diffraction XCALIBUR four-circles X-ray diffractometer using graphite monochromatized MoKα radiation (λ = 0.7173 Å) equipped with a SAPPHIR CCD two-dimensional detector. The intensity data were corrected for Lorentz and polarization effects. A numeric analytical absorption correction was carried out with the program CrysAlis RED27. The metal and phosphorus atoms were located by direct methods, using the SHELXS-97 program28, while the remaining atoms were found from successive Fourier difference maps. Atomic positions were refined by full matrix least-squares method using SHELXL-97 program29. Full-matrix least squares refinement was based on F2. Thermal displacements of all non-hydrogen atoms were refined anisotropically. Pertinent crystallographic details are given in Tables 1, 2 and 3. Graphics have been performed using DIAMOND program30. Tables of crystal structures and refinements, notably full bond lengths and angles, and anisotropic thermal parameters have been deposited with the Inorganic Crystal Structure Database, FIZ, Hermann von Helmholtz Platz 1, 76,344 Eggenstein Leopoldshafen, Germany; fax: (+ 49) 7247 808 132; Email: crysdata@fiz-karlsruhe.de. CSD-deposition numbers are respectively 421807 for Rb2Co(H2P2O7)2·2H2O.

Table 1 Crystal data and structure refinement for Rb2Co(H2P2O7)2·2H2O.
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Table 2 Atomic coordinates and isotropic displacement parameters Ueqa for Rb2Co(H2P2O7)2·2H2O.
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Table 3 Bond lengths (Ǻ) values in the structures of Rb2Co(H2P2O7)2·2H2O.
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Gas adsorption/sorption device

The absorption/desorption measurements of carbon dioxide (CO2) and methane (CH4) with the synthesized material were successfully performed at temperatures of 298 and 318 K, and from vacuum to 50 bars, by using the specific high pressure “magnetic suspension sorption apparatus (MSA)” equipment, purchased from Rubotherm Präzisionsmesstechnik GmbH. This tool has the ability to raise the pressure up to 350 bars, and temperature up to 373 °K. The pressure transducers (ParoscientificTM, USA) were installed to measure the pressure from vacuum to 350 bars with an accuracy of 0.01% bar, and temperature sensor (Minco PRT, USA) with a sensitivity of ± 0.5 °C. The detailed experimental procedure and calibration are detailed in elsewhere31,32,33.

Density functional theory

Computer simulations are conducted using DFT within the generalized gradient approximation of Perdew–Burke–Ernzerhof (PBE) to describe the exchange–correlation energy34. The plane-wave basis set with a cut-off energy of 1360.57 eV is used to describe the atoms in the system. The Brillouin zone integration was conducted using 15 × 15 × 15 Monkhorst–Pack k-points sampling35 for the unit cell structure of the material. The convergence criterion for Hellman–Feynman forces was 0.01 eV/Å. Interactions of CO2 and CH4 molecules with the surface of the material is characterized by the electron difference density which is defined as the difference between the self-consistent valence charge density and the superposition of atomic valence densities36. Simulations are conducted using the computational package Atomistix toolkit37,38.

Results and discussions

Crystal structure study

The structural model in Rb2Co(H2P2O7)·2H2O consists of a 3D framework, made from corners and edges sharing of [RbO7], [H2P2O7] and [CoO6] entities. Figure 1a,b depict the shape of the crystal structure of the title compound onto the crystallographic (010) and (001) planes. H-bonds are represented by dashed lines. These figures have been drawn according to the respective atomic coordinates reported in Table 2 and to the bond lengths and angles represented in Table 3. The structure might be described in terms of isolated [CoO4(H2O)2(H2P2O7)2] units which propagates tri-dimensionally through Rb–O interactions and an intricate H-bonds network between the H2P2O7’s hydroxyl groups. Such unit [CoO4(H2O)2(H2P2O7)2] results from the edge sharing [CoO4(H2O)2] octahedron and [H2P2O7] double tetrahedrons. The unit cell contains two distinctive phosphorous (V) sites coordinated by four (04) oxygen atoms in a slightly distorted tetrahedron. Two (02) tetrahedra share an apex to form the dihydrogenpyrophosphate group H2P2O7. Average <P‒O> distance is about 1.551 Å. This value is similar to its homologous distances in the known isoformular compounds (NH)2Co(H2P2O7)2·2H2O21. As in the other mixed pyrophosphates, H2P2O7 shows almost an eclipsed conformation. The ∠(P–O–P) bridging angle is one feature characterizing the [H2P2O7]2- anion, here of 129.3°. In this new pyrophosphates, Co2+ cations are placed on inversion centre position (0, 1/2, O). Their octahedral coordination is composed of four (04) Oxygen atoms from two (02) bidendate [H2P2O7] groups and the remaining two (02) Oxygen from the water molecules. However, the M2+ cation in the orthorhombic form of K2M(H2P2O7)2·2H2O (M=Co, Ni, Cu)22 is located in the mirror plane within an octahedral coordination formed by two (02) bidendate [H2P2O7]2- counter ions and two (02) water molecules. Average \({\overline{\text{d}}}\)(Co‒O) is of 2.095 Å, value was compared with recently the one reported value of (NH4)2Co(H2P2O7)2·2H2O (2.053 Å) (Table 3)21.

Figure 1
figure 1

(a, b) Rb2Co(H2P2O7)2·2H2O as representative crystal structure, projections of the framework onto crystallographic (010) and (001) planes. H-bonds are represented as dashed lines.

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UV–Vis study

The UV–Visible spectrum of the Rb2Co(H2P2O7)·2H2O is displayed in Fig. 2a,b. It might be described in terms of electronic transitions between the ground state 4T1g and the upper ones 4T2g, 4A2g and 4T1g(P). In fact, three spins allowed transitions may be expected. The lowest energy band observed near 7109.6 cm−1 is assigned to the 4T2g → 4T1g transition, while the main absorption band in the visible region near 14,581 cm−1 is rather assigned to 4T2g (P) → 4T1g.

Figure 2
figure 2

UV–Visible spectra of the Rb2Co(H2P2O7)2·2H2O compound.

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CO 2 and CH 4 adsorption and desorption

The ultimate aim of the present work is to experimentally investigate the sorption capacity of CO2 and CH4 onto the Rb2Co(H2P2O7)·2H2O. For both gases, the experiments were performed at temperatures of 298 and 318 K, respectively. At each temperature, cycles of adsorption and desorption were run by stepwise increment/decrement process of CO2 and CH4 pressures, ranging from vacuum to 50 bars and its vice-versa. At each temperature, individual cycles of CO2 and CH4 were recorded that consist of 12 adsorption and 8 desorption points with Rb2Co(H2P2O7)·2H2O, illustrated in supplementary Tables S1 and S2. Similarly, all the values were used to plot the figures to understand the behavior (hysteresis and physisorption) of Rb2Co(H2P2O7)·2H2O at moderate and higher temperature from vacuum to high pressure through a programmed adsorption–desorption cycle in Figs. S1–S4. On looking over the plots (Figs. S1–S4) of their corresponding gas at both the temperature, figures demonstrate a smooth trend of variation with respect to pressure by overlapping adsorption and desorption points. These collective adsorption–desorption trends indicate that there are no hysteresis, and no significant changes occur in material. Additionally, it has to be mentioned that in order to obtain reliable data, peripheral conditions like humidity, ambient pressure, and temperature were taken care of 39. The similar trend of variation (adsorption and desorption pressure point that lies at same plotted line) have found in our recently published work with (NH4)2 Mg(H2P2O7)2·2H2O) with CO2 and CH4 at 298 and 318 K 40 Figs. S1, S2 and S3 show a common thermodynamic trend of variation; solubility of gas decreases when increasing the temperatures, and increases when increasing the pressures which in turns lead to increase the adsorption capacity of CO2 and CH4 in m.mol per gram of sample. Although, similar trend of adsorption/desorption indicates physisorption of gases gives the advantage of reusability for energy and economic saving 41,42 point of view as ionic liquids and deep eutectic solvent being tested as liquid absorbent 33,39,42,43,44. Consequently, this efficient crystal could be very useful materials for one of flue gas process treatment stages. Furthermore, the trend of CO2 and CH4 adsorption shows that our new crystal could be easily regenerated in an efficient process. A close scrutiny of adsorption results shows that CO2 (3.1016 m mol/g) sorption is higher than CH4 (2.3467 m mol/g) at temperature 298 K and at pressure of 50 bars, where maximum sorption was expected. This comparison gives clear evidence of a selective behavior of the crystal acidic pyrophosphate Rb2Co(H2P2O7)·2H2O for CO2 and CH4 gases at each temperature and pressure. On further reviewing Figs. 3 and 4 single crystal sample shows that the sorptivity of CO2 is higher than that of the CH4 at 318 K temperatures, additionally, ~ 0.5 bar sorptivity difference between both gases at both the temperatures at its highest measured pressure. However, this is the continuation of our recent published research work 40 and comparative plot of present and previously investigated has been included in supporting information as Fig. S5. Previously used material ((NH4)2 Mg(H2P2O7)2·2H2O) for CO2 and CH4 at 50 bar, and 298 K and 318 K values shows that adsorb 3.3858 m mol/g, 2.9657 m mol/g of CO2 of (NH4)2 Mg(H2P2O7)2·2H2O, respectively, at the same pressure and temperatures 5.3234 m mol/g and 8.2909 m mol/g respectively for CH4. The CO2 sorption is almost close to the currently investigated sample however CH4 sorption values are 3–4 fold higher in comparison with Rb2Co(H2P2O7)·2H2O at temperatures. Moreover, on comparing CO2 sorption data of this work and hydroxy metal carbonates M(CO3)x(OH)y (M=Zn, Zn–Mg, Mg, Mg–Cu, Cu, Ni, and Pb) 45, found that Rb2Co(H2P2O7)·2H2O shows higher values 35 bar at 318 K, although hydroxy metal carbonates were measured at 316 K. The enhanced CO2 sorption capacity of Rb2Co(H2P2O7)·2H2O compared to hydroxy metal carbonates may be attributed to its unique three-dimensional framework. The presence of [RbO7], [H2P2O7], and [CoO6] entities interconnected through a dense network of hydrogen bonds can offer specific adsorption sites and channels, facilitating better gas uptake. Furthermore, the preferential adsorption of CO2 in Rb2Co(H2P2O7)·2H2O over hydroxy metal carbonates can be intricately linked to the electronic nature of its components. The coordination of the Co ion, often known for its variable oxidation state and magnetic properties, combined with the phosphate groups, may provide an electron-rich environment. This can enhance the Lewis acid–base interactions with the electrophilic carbon of CO2, resulting in better sorption. Additionally, the dense hydrogen-bonding network might create a dynamic environment, allowing for reversible adsorption and facilitating efficient gas uptake and release. In Fig. 5, we have demonstrated the CO2/CH4 ideal selectivity results at 298 K and 318 K from pressure 1–50 bar (11 sorption data point). The CO2/CH4 (mmol gas per g of sample) ideal selectivity at 298 K was higher as compared to 318 K, as expected, and trends of variations are close to each other at both the temperatures. Furthermore, after 35 bar significantly increased values were noticed, and difference was reached at maximum investigated pressure. Last but not the least, CO2/CH4 ideal selectivity of Rb2Co(H2P2O7)·2H2O is much higher than (NH4)2Mg(H2P2O7)2·2H2O of previous work 40 at all the pressures and temperatures. CO2, with its linear molecular geometry and inherent quadrupole moment, interacts differently with adsorbents compared to tetrahedral CH4. Rb2Co(H2P2O7)·2H2O might provide specific sites where the CO2 quadrupole can align favorably, leading to enhanced selectivity. Rb’s relatively larger ionic size may influence the pore geometries and accessibility, leading to differential interactions with CO2 and CH4 molecules, thus affecting the separation efficiency. Furthermore, due to these large ionic radii for Rb and diffuse electron cloud can also lead to induction of more significant dispersion forces. This, combined with the potential electronic interactions from Co and phosphate entities, might selectively favor the adsorption of CO2 over CH4, fine-tuning the separation efficiency.

Figure 3
figure 3

CO2 absorption trends in Rb2Co(H2P2O7)2·2H2O at 298 and 318 K, and different pressures.

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Figure 4
figure 4

CH4 absorption trends in Rb2Co(H2P2O7)2·2H2O at 298 and 318 K and different pressures.

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Figure 5
figure 5

CH4/CO2 ideal selectivity at low and high pressures for studied Rb2Co(H2P2O7)2·2H2O at 298 and 318 K and different pressures.

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Simulation results for CO2 and CH4 adsorption

We started by creating and optimizing the unit cell structure of the Rb2Co(H2P2O7)·2H2O material using our experimental findings. Figure 6 shows the unit cell structure of the material after geometry optimization. The optimized structure has a triclinic lattice with parameters a = 7.008 Å, b = 7.44 Å, and c = 7.71 Å, and angles α = 82.24, β = 72.44 and γ = 86.42. Using this unit cell structure, we have constructed and further optimized 21 slab geometries with 5 different surface symmetries (100, 001, 010, 101, 011, 110, and 111) and 3 different terminations (P-, Ru-, and Co-terminations). Total energy calculations46,47 show that a slab with Co-terminated 111 surface has the lowest energy for this material. Next, we study the adsorption of CO2 and CH4 molecules on this surface. To characterize the adsorption properties of the material, we have conducted total energy and electron localization function calculations. The adsorption energies are calculated as:

$${E}_{ads}={E}_{(slab+mol)}-{E}_{slab}-{E}_{mol},$$

where Emol is the total energy of the isolated molecule and Eslab is the total energy of the slab in the absence of the molecule.

Figure 6
figure 6

Optimized unit cell structure of Rb2Co(H2P2O7)2·2H2O.

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To find the adsorption sites with minimum energy, we have started our calculations for 15 different locations and orientations of the molecules on Co-terminated 111 surface of Rb2Co(H2P2O7)·2H2O. The atoms at bottom layer of the slab kept fixed during structural optimization to represent the bulk of the material. Figure 7 shows the lowest energy configurations for CH4 (a) and CO2 (b) molecules. CH4 molecule is physisorbed on the surface of the slab near the Co atom and therefore remains unperturbed (see Fig. 7a). The calculated adsorption energy of this molecule is − 0.42 eV. CO2 molecule is adsorbed more strongly to the surface with adsorption energy of − 0.93 eV. The electronic structure analysis shows the covalent bond formation between the molecule and Co atom of the material (Fig. 7b). Indeed, we obtained a strong overlap of the electron localization function between the molecule and the substrate. Thus, DFT calculations also reveal strong gas adsorption properties of the synthesized Rb2Co(H2P2O7)·2H2O material.

Figure 7
figure 7

Lowest energy configurations for CH4 (a) and CO2 (b) molecules on Co-terminated 111 surface of Rb2Co(H2P2O7)2·2H2O. The figure also shows the isosurface plots (isovalue ± 0.1 Å−3) of the electron difference density.

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Conclusions

The new pyrophosphate Rb2Co(H2P2O7)·2H2O compound was successfully synthesized in a solution-like and was found to crystallize in the triclinic system, space group (\(P\overline{1}\)). Its UV–Vis spectrum has shown the usual transitions between the ground state 4T1g and the supper levels 4T2g, 4A2g and 4T1g(P). The sorption results clearly revealed that the prepared compound manifested a high adsorption capacity of the CO2 and CH4 gases at different pressures and temperatures. By increasing the temperature, the sorption capacity decreased and increases with respect to the pressure, leading to high sorption values which are quite normal thermodynamically. However, a common adsorption/desorption track of pressure trends alludes to the physisorption phenomena. More interestingly, gas sorption comparison study showed clear evidence of a selective behavior of the acidic pyrophosphate Rb2Co(H2P2O7)·2H2O crystal for CO2 and CH4 gases at each temperature and pressure, and revealed also that the sample adsorbtivity of CO2 is more than that of the CH4 at different temperatures and pressures. Computer simulations also reveal the selectivity of the material to CO2 molecules as compared to methane molecules.