Single crystal structure, vibrational spectroscopy, gas sorption and antimicrobial properties of a new inorganic acidic diphosphates material (NH4)2Mg(H2P2O7)2•2H2O

We report on the successful synthesis of diammonium magnesium dihydrogendiphosphate (V) dihydrate compound (NH4)2Mg(H2P2O7)2•2H2O using a wet chemical route. Single crystal X-ray diffraction analysis and micro Raman spectroscopy are employed to characterize the compound. We demonstrate, using a multidisciplinary approach, that this compound is ideal for carbon dioxide (CO2) capture in addition to other anthropogenic gasses. We show here -from both an experimental as well as from a density functional theory (DFT) calculations routes- the potential for adopting this compound into domestic air-conditioning units (ACUs). From these experiments, the resistance to bacterial growth is also investigated, which is critical for the adoption of this compound in ACUs. Our compound exhibits a higher methane (CH4) sorptivity as compared to CO2 at 25 °C and 45 °C under pressures up to 50 bars. Furthermore, DFT electronic structure calculations are used to compute the main structural and electronic properties of the compound, taking into consideration the characteristics of the identified pores as a function of the progressive CO2 vs. CH4 loadings. Finally, the antibacterial assay reveals a strong antibacterial activity against the tested Gram-positive and Gram-negative bacteria, with a large zone of inhibition against the tested E. Coli, S. Aureus and K. Pneumonia.


Results and Discussion
X-ray crystal structure. ( Fig. 1a,b). The Mg atom lies on the inversion center (0, 1/2, 0) and is octahedraly coordinated by four O atoms from two bidendate H 2 P 2 O 7 groups, and two water molecules in trans positions with respect to the basal plane containing phosphorous and magnesium atoms (Fig. 1b). Selected interatomic distances, angles and bond valences of (NH 4 33 . In the ammonium cation, the average N-H distance is 0.897 Å, with the hydrogen pointing toward oxygen atoms of adjacent (H 2 P 2 O 7 ) 2 groups, forming a network of hydrogen bonds (see Table S6). Effectively, considering the H-bonds, [Mg(H 2 P 2 O 7 ) 2 •2H 2 O] 2anions turn in fact not isolated, they are interconnected through three types of moderate hydrogen bonds (i.e. P-O-H … O-P, Mg-O-H … O-P, and N-H … O-Mg) and form a stable 3D-framework. These hydrogen bonds include the H atoms from the ammonium cation (NH 4+ ) (Fig. 2a), the hydroxyl groups (PO 3 OH) (Fig. 2b), and the water molecules (H 2 O) (Fig. 2c). Figure 3a shows a representative SEM micrograph of the (NH 4 ) 2 Mg(H 2 P 2 O 7 ) 2 •2H 2 O compound and (b-g) associated EDS elemental maps with elements O, C, P, Mg and N, respectively. Figure 4 shows the EDS spectrum of (NH 4 ) 2 Mg(H 2 P 2 O 7 ) 2 •2H 2 O. The atomic concentrations are shown Table S1.
Vibrational spectroscopy. Factor group analysis. The Raman and infrared spectra of this compound have been collected and interpreted using factor group analysis. The crystallographic study shows that Mg cations are located on the 1c site, all the other ions are located on the (2i) sites. The irreducible representation of the compound in the C i factor group (excluding 3 acoustic modes) leads to 57A g (Ra) + 57A u (IR) modes. The factor groups are centrosymmetric, the rule of mutual exclusion applies: the lines which are active in IR are not in Raman and vice versa. Note that Ag modes are Raman active and Au ones are infrared active.
The factor group analysis predicts the distribution of irreducible representation of the internal modes of (H 2 P 2 O 7 ) 2-, (NH 4 ) + ions and H 2 O molecules in the unit cell of the crystal (NH 4 ) 2 Mg (H 2 P 2 O 7 ) 2 •2H 2 O, to be respectively as follows: Tables S7-S11 show the origin as well as a summary of the infrared and Raman activity of the internal and external modes of (H 2 P 2 O 7 ) 2-, H 2 O and (NH 4 ) + , and Mg 2+ in (NH 4 interpretation of the Raman and infrared spectra. The interpretation of the Raman (Fig. 5) and infrared ( Fig. 6) spectra can be made on the basis of characteristic vibrations of PO 2 group, P-OH bond, P-O-P bridge, H 2 O and (NH 4 ) + groups [10][11][12] . In both spectra, six strong bands are located at: 3744, 3346, 3276, 3110, 1670, 1446 and 1333 cm −1 , while broad bands located in the region between 3744-3346 cm −1 correspond to the stretching vibration of the two water molecules (v(H 2 O)). The band falling in the region 3276-3110 cm −1 is associated to the v(NH 4   www.nature.com/scientificreports www.nature.com/scientificreports/ γas(P-O-P) = 904 and 968 cm −1 , γs(P-O-P) = 713 cm −1 and 755 cm −1 , and three others in infrared spectrum at: γas(P-O-P) = 936 and 992 cm −1 , γs(P-O-P) = 744 cm −1 , which confirms the low symmetry of the cell (γ is the symmetric and/or asymmetric valence vibration modes) [10][11][12][13][14][15][16][17]34 . The band located at 849 cm −1 in Raman spectrum is due to the P-OH mode [10][11][12][13][14][15][16][17] . The presence of γas(P-O-P) in the infrared spectrum and γas(P-O-P) in the Raman spectrum leads to a bent P-O-P bridge angle 34 . In the Raman spectrum, the modes lying between 239 and 384 cm −1 can be attributed to the external, torsional and P-O-P deformation modes. The δ(P-O-P) is observed at 315 cm −1 , while the rocking vibration mode of the PO 2 and the P-OH deformation mode are observed in the www.nature.com/scientificreports www.nature.com/scientificreports/ 300-594 cm −1 region 35 . A comparison of the Raman and infrared spectra shows that most of the bands do not coincide, which confirms a centrosymmetric structure of the (NH 4 ) 2 Mg (H 2 P 2 O 7 ) 2 •2H 2 O. co 2 and cH 4 sorption. The absorption and selectivity of (NH 4 ) 2 Mg(H 2 P 2 O 7 ) 2 •2H 2 O are evaluated by means of gravimetric analysis of the adsorption and desorption of CO 2 and CH 4 gases, which provides relevant information on the chemisorption versus physisorption behavior. Tests were performed at selected temperatures, namely 25 °C and 45 °C, while the pressure was varied from 0 to 50 bars. A stepwise absorption-desorption cycle was conducted by a programmed interval at each temperature. Buoyancy corrections on the sorption results -which operated through in-situ density measuring ability of magnetic suspension sorption apparatus (MSA) -were included. Additionally, external environmental conditions (humidity, ambient pressure and temperature) were www.nature.com/scientificreports www.nature.com/scientificreports/ considered automatically by MSA for zero-point corrections, to be measured with high accuracy to provide an authentic data set. Interestingly, no chemisorption/hysteresis was observed throughout our experiments as could be seen from the plots in Fig. 7 indicative of a preferred CO 2 and CH 4 physisorption of this material. The corresponding absorption-desorption values have been tabulated and made available in the supporting information (see Table S14 and Figures S1-S4). The first noticeable remark is that the sorption increases with increasing pressure for both CO 2 and CH 4 gases. We recorded a maximum sorption of 8.29 mmol/g for CH 4 at 45 °C and 50 bar while a lower value of 5.32 mmol/g at 25 °C was recorded at the same pressure. Lower sorption was recorder for CO 2 with a maximum value not exceeding 2.96 mmol/g at 45 °C and 50 bar while a higher value of 3.38 mmol/g is measured at 25 °C. Atilhan and coworkers 36 found values of the same order of magnitude by using magnetic suspension balance (MSB) system on Montmorillonite Nanoclays. The highest reported adsorption performance of CO 2 versus CH 4 at a temperature of 25 °C and a pressure of 50 bar was 3.47 mmol/g versus 3.23 mmol/g respectively. Interestingly, our material presents highly reproducible trends of high sorptivity of CH 4 compared to CO 2 at ambient temperature. Differences in CH 4 vs. CO 2 sorptivity become more pronounced with increasing temperature, a phenomenon deserving further experimental investigation to identify the exact mechanism governing it. For a comparison purpose, we have tested the CO 2 and CH 4 gas sorption at two different temperatures (25 &     www.nature.com/scientificreports www.nature.com/scientificreports/ in agreement with the experimentally reported structural data as displayed in Table S4. The pristine material is characterized by several pores with different coordination and radii as shown in Fig. 9a. A large pore (labeled 1) has a diameter of 3.4 Å forming a one-dimensional channel along the a direction. The spherical shape of pore 1 would be favorable to adsorb both CO 2 and CH 4 as shown in Fig. 9b. The subsequent pore 2 has a diameter of 2.9 Å forming a one-dimensional channel along the b direction as shown in Fig. 9c. Pore 3 shown in Fig. 9d is much smaller, with a diameter of 2.2 Å, and extends along the c direction. Another pore located in the center diameter 0.8 Å could be identified, however its elongated shape cannot accommodate large spherical molecules. Nevertheless, it could accommodate elongated molecules such as CO 2 , however the chemical environment and the coordination need to be favorable as well. Table S11 gives details about the identified pores with diameter> 2 Å. Combining this understanding of the morphology of the available pores to load molecules and the identified possible molecular diffusion pathways favor the hypothesis that pore 1 shall be loaded at first hand. We adopt an approach where we reassess the availability of open pores after each molecular loading. This is mainly motivated by our accurate understanding of the crystal structure featuring hydrogen bonds that are sensitive to the interaction molecules while -at the same time-giving an enhanced flexibility of bonding between the octahedral which is an advantage of this hybrid material. Thus, we cannot rule out that pore 1 could grow after the first loading and becomes favorable for accepting more molecules especially because of its 1D channel of diffusion parallel to the a direction. Figure 10 shows the calculated band structure of the pristine (NH 4 ) 2 Mg(H 2 P 2 O 7 ) 2 •2H 2 O. The system is an insulator with an indirect gap of 5.37 eV. The valence band maximum (VBM) is located near the R point displaying weakly dispersive flat bands. The conduction band minimum (CBM) is located at Γ point with well dispersed bands indicative of a strong overlap between molecular orbitals. Turning to the electronic density of states, Fig. 10a shows that Mg states contribute weakly to both VB and CB and far from the band edges. However, O 2p orbitals dominated the VBM with some contribution form P 3p orbitals while the conduction band is hybridized between the O 2p and P 3p orbitals with marginal contribution coming from other elements. The calculated real (ε1) and imaginary part (ε2) of the complex dielectric function are given in Fig. 11 also the Bader valence electron charges, charge transfer (relative to atoms), Bader volumes and the atomic partial charges in units of electron charges for DFT relaxed (NH 4 ) 2 Mg(H 2 P 2 O 7 ) 2 •2H 2 O are made available in Tables S15 and S16, respectively.
We proceeded by loading (NH 4 ) 2 Mg(H 2 P 2 O 7 ) 2 •2H 2 O with CO 2 and CH 4 molecules by targeting every time to occupy the largest available pores. In the case of the unloaded materials, each of the pore 1 sites (as shown in Fig. 9b) are occupied with a CO 2 or a CH 4 molecule.
We loaded up to four CH 4 molecules to occupy the large pore located between the P1 and P2 tetrahedra (pore 1), using the procedure described above. After each loading, the largest pore is identified then subsequently occupied as shown in the sequence illustrated in Fig. 12. It is noticeable from Fig. 12 that additional P-O … H-C hydrogen bonds form between the loaded CH 4 molecule and the oxygen forming the PO 4 tetrahedra. This newly formed hydrogen bonds in addition to the ones present in the pristine material (see Table S5) offers a significant flexibility of the material and might explain the increased capacity of loading CH 4 by increasing temperature. We observe that loading the first two CH 4 molecules allows to enlarge the diameter of pore 1 from d pore1 = 3.4 Å to d pore1 = 4.38 Å at the expense of slightly shrinking pore 2 and pore 3. Hence, it is more favorable to still load the third CH 4 molecule within pore 1. Upon 3 CH 4 molecules loading pore 1 reaches its maximum capacity, hence the next available pore to adsorb CH 4 is pore 2 with a diameter d pore2 = 2.9 Å and diffusion pathway along the b-direction. We proceed similarly to load the first two CO 2 molecules at pore 1, it is noticeable that the CO 2 molecule align to form Mg-O-H … O-C hydrogen bonds with the [MgO 4 (H 2 O) 2 ] octahedra avoiding the electrostatic repulsion originating from the neighboring PO 4 tetrahedra. Both the dipole moment and the electrostatic directionality of the bonding suggest it pore 1 might not be able to accommodate further CO 2 molecules. Indeed, after subjecting materials already loaded with two CO 2 molecules to available pore searches, we identify the next loading position to be pore 2 with d pore2 = 3.0 Å. Additional loading with CO 2 in the vicinity of PO 4 tetrahedra seems to be energetically unfavorable as it of suffers from a strong electrostatic repulsion with the CO 2 oxygen atoms. We loaded a third CO 2 molecule which seems on average to adsorb less strongly to the pore 2.
Upon progressive addition of molecules to the largest available pore or so-called loading, it is noticeable that the material is more favorable to the adsorption of CH 4 than CO 2 . Hence the sorption with CH 4 is more favorable than that of CO 2 in agreement with the experimental finding reported in the previous section. In light of our DFT calculations, the large sorption with CH 4 can be attributed to formation of hydrogen bonds between the molecule and the PO 4 tetrahedra contributing in bridging additional flexibility to the overall hydrogen bonding network. In contrast, CO 2 loading is highly directional, as such CO 2 molecule form hydrogen bonds with the [MgO 4 (H 2 O) 2 ] octahedral while -at the same time-suffering from electrostatic repulsion with the PO 4 tetrahedra which might be at the origin of the decreased sorption with CO 2 observed experimentally. We speculate that this repulsion becomes more pronounced by increasing temperature from 25-45 °C as observed experimentally. Although our calculations do not explicitly take into account the increase in temperature and pressure, assuming that under experimental loading conditions the properties of the pores and the atomic vibration are comparable regardless of the loaded molecule, the above finding remains valid.
Antibacterial activity. The antibacterial assay revealed that (NH 4 ) 2 Mg(H 2 P 2 O 7 ) 2 •2H 2 O compound exhibits strong antibacterial activity against the tested Gram-positive and Gram-negative bacteria. According to Fig. 13 and Table S12, this material showed about 15 mm zone of inhibition against E. Coli at 5 mg, similarly, the same  X-ray crystallography. A crystal of (NH 4 ) 2 Mg(H 2 P 2 O 7 ) 2 •2H 2 O, rounded block 0.20 mm × 0.14 mm × 0.10 mm, was glued to a thin glass fiber and mounted on a OXFORD DIFFRACTION XCALIBUR four-circle X-ray diffractometer, equipped with graphite monochromatic MoKα radiation (λ = 0.7173 Å) and equipped with a SAPPHIR CCD two-dimensional detector. A total of 3014 reflections were collected (2θ max = 26.37°) using the ω = 2θ scan mode. Of these 1905 are unique and 1587 were considered observed I > 2σ (I). The intensity data were corrected for Lorentz and polarization effects. A numeric analytical absorption correction was carried out with the program CrysAlis RED 37 Most of the positions of magnesium, phosphorus and oxygen atoms were located by direct methods, using the SHELXS-97 program 35 , and the remaining atoms were found from successive Fourier difference maps. Atomic positions were refined by fullmatrix least-squares method using SHELXL-97 program 38 . The non-hydrogen atoms were refined anisotropically. The H atoms were located geometrically, and attributed isotropic thermal factors equal to 1.2 those of the atoms on which they are linked. A final cycle refinement including atomic coordinates and anisotropic thermal parameters converged at R(F) = 0.0483 and wR(F 2 ) = 0.1305 for the observed reflections. The unit cell parameters and data collection details are presented in Tables S2 and S3, respectively. The refined atomic positions and anisotropic ADPs are given in Tables S4 and  S5, respectively. The structural graphics were created with DIAMOND program 39 . Further details on the structure refinements of (NH 4 ) 2 Mg(H 2 P 2 O 7 ) 2 •2H 2 O may be obtained from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen (Germany), by quoting the Registry No. CSD-1988943. Raman spectroscopy. Details on the Raman spectroscopy can be found in our ref. 12 . Briefly, the Raman spectroscopy was performed in air at room temperature, spectrum of (NH 4 ) 2 Mg(H 2 P 2 O 7 ) 2 •2H 2 O was collected in a back-scattering configuration via high throughput holographic imaging spectrograph equipped with volume www.nature.com/scientificreports www.nature.com/scientificreports/ transmission grating, holographic notch filter and thermoelectrically cooled CCD detector (Physics Spectra). The acquisition resolution was about 4 cm −1 . Ti 3+ -sapphire NIR laser pumped by an argon ion laser was tuned at 785 nm. The laser was operated at a power not exceeding 40 mW to avoid any degradation of the samples, and the exposure time was about 60 s and 10 accumulations. A PERKIN-ELMER 1750 spectrophotometer was used for the infrared absorption analysis, in the 400-4000 cm −1 range.

Gas adsorption-desorption analysis. Rubotherm magnetic suspension sorption apparatus (MSA)
allows the changes in force and mass which act on the sample that basically work on Archimedes' buoyancy principle from high pressure to vacuum in order to complete gas absorption-desorption experiments. The data correlation, magnetic suspension force transmission error correction, calibration and working principle and methodology associated paraphernalia for broad range of adsorbents and absorbents were explained in detail in previous studies 40 . This apparatus contains in-situ density measurement capability with 4 kgm −3 uncertainty that makes possible for direct gravimetric measurements during the sorption experiments. Assembled pressure transducers (Paroscientific, USA) and temperature sensor (Minco PRT, USA) are capable to measure pressure up to 350 bars with an uncertainty of 0.01% of the full scale (u(p) ≈ 0.035 bar), and temperature with accuracy of ± 0.5 K (u(T) = 0.05 K), respectively. The automatic operated sorption apparatus starts measuring from vacuum to achieve high pressure through each pressure interval (as per given protocol) until the equilibrium is reached typically 45 to 60 min. MSA is operated to measure low pressure towards high pressure (adsorption) and then from high pressure to low pressure (desorption) including the end-of-sorption-cycle vacuum point measurement to be sure for chemisorption existence during the sorption cycle. The studied sample was well dried desiccated prior to use, and further vacuum was applied for at least 10 h before the sorption cycle initiated. The unchanged sample in the measurement bucket has given gas absorption-desorption data at isotherms of 25 and 45 °C.
Microorganisms and inoculum preparation. The antimicrobial properties of the (NH 4 ) 2 Mg(H 2 P 2 O 7 ) 2 •2H 2 O compound were evaluated using staphylococcus aureus as a model of Gram-positive  www.nature.com/scientificreports www.nature.com/scientificreports/ bacteria and by using two other Gram-negative bacteria, namely Escherichia Coli and Klebsiella Pneumonia. For inoculum preparation, a single bacterial colony was picked from nutrient agar using disposable sterile loop, transferred into 10 mL of nutrient broth and placed overnight in incubator shaker at 37 °C with shaking speed of 100 rpm. The bacterial cell density was measured at an optical density (OD) of 600 nm using a spectro-photometer, each inoculum prepared would contains approximately 107 cfu/ml of bacteria. Bacterial sensitivity to (NH 4 ) 2 Mg(H 2 P 2 O 7 ) 2 •2H 2 O compound is performed by employing agar well diffusion method. Three-millimeter diameter holes were made in the agar plates using 50 ml disposable pipette and different concentration varying from 5 to 20 mg of this material was placed carefully in the holes, Ampicillin (10 µg/ml) was used as a standard antibiotic. The plates were overlaid with a mixture of each bacteria with 2 ml of molten 1.5% (w./vol.) noble agar (Sigma-Aldrich) at proximately 65 °C. Finally, the plates were incubated at 37 °C for 24 h and the average diameter of the inhibition zone surrounding the holes was examined.
Density functional theory (Dft) calculations. Density functional theory (DFT) calculations were employed to study the properties of (NH 4 ) 2 Mg(H 2 P 2 O 7 ) 2 porous material. Spin polarized DFT GGA calculation was used the Perdew-Burke-Ernzerhof functional (PBE) as implemented in the Vienna ab-initio simulation package [41][42][43][44][45][46][47] (VASP) with the projector augmented wave (PAW) pseudopotentials. Due to the presence of hydrogen bonds, Van Der Waals (vdW) interactions where taken into account via Tkatchenko-Scheffler (TS) scheme 48,49 . Full structural optimization of the unit cell was performed until convergence criteria for optimizations reached 10 -3 eV and 10 -4 eV for the ionic relaxation loop and self-consistent electronic iteration, respectively. A kinetic cutoff energy of 520 eV for the plane waves has been employed and the Brillouin zone was sampled by Monkhorst-Pack grid centered at the Gamma point with a k-point mesh of 8×8×8. conclusion diammonium magnesium dihydrogendiphosphate (V) dehydrate (NH 4 ) 2 Mg(H 2 P 2 O 7 ) 2 •2H 2 O was successfully synthesized by a wet chemical process, and characterized in detail by X-ray diffraction, FTIR and micro Raman spectroscopy. This compound was found to crystallize in the triclinic system, space group P−1 (No.2) with a = 7.007 (9)  The compound was then tested for CO 2 and CH 4 storage applications and showed higher sorptivity with CH 4 than with CO 2 at 25 °C and 45 °C under pressures up to 50 bars. DFT calculations were also employed to compute the principal characteristics of identified pores in this compound, including the projected electronic density of states, band structures, the complex dielectric function, as function of the progressive CO 2 vs. CH 4 loadings. An antibacterial assay was also performed and has shown a strong antibacterial activity against the tested Gram-positive and Gram-negative bacteria, with a large zone of inhibition against the tested E. Coli, S. Aureus and K. Pneumonia, demonstrating thereby a real potential for preventing the proliferation of infectious diseases. The multidisciplinary analysis presented in this work demonstrated clearly the potential of this material for the selective capture of CH 4 /CO 2 gases opening the way for its applications in conjunction with carbon capture from air conditioning systems in houses/buildings and for energy applications at large (e.g. liquid hydrocarbon fuels production).