Investigating CO2 storage properties of Pd(II) and Co(II) chelates of a Schiff's base ligand

A new metal complexes are made from the ligands derived from amoxicillin based Schiff's base coordinated with Pd(II) and Co(II) have been synthesized and characterized via different spectroscopic methods. FT-IR spectroscopy have shown a formation of tetrahedral and square planar geometry for Co(II) and Pd(II) complexes, respectively. Surface morphology was inspected via field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). The Brunauer–Emmett–Teller surface area of the metal complexes samples is about 6.63 to 8.71 m2/g, with pore diameters and volume of 0.030–0.0501 cm3/g and 18.39–22.98 nm, respectively. The quadrupole moment of CO2 has a great effect on the adsorption capacity as it is highly diffusible of 21.38, 26.16 cm3.g−1 for Co(II) and Pd(II) complex.


Introduction
Energy storage as gas is one of the best ways to successfully meet energy demand [1,2]. Different methods are developed to meet the increase demand for energy with sustainable impact to our environment by reducing the carbon emission associated with the process of producing energy. Biomass, solar and wind are environmentally friendly sources of energy with low carbon emission [3,4]. By using only these sources, CO 2 emissions principle be reduced to zero [3,5]. However, the cost, geographical limitation, year-round continuous availability, and storage capacity are limiting the use of renewable energy sources [6]. One of the most used technologies is capture and storage of CO 2 gas [7]. Ethanol amine process is proven to be a good adsorbent for CO 2 but a very costly process since there is a need for high energy as well as the use of evaporative materials [8]. Thus, there is a need for alternative chemisorbents that are easy and cheap to manufacture and make the adsorption process economical [9]. The main criteria of CO 2 adsorbent materials are good adsorption capacity, high efficient and durable in term of reusability and regeneration of such materials [10]. Materials including zeolites, activated carbon, and silicic acid are tested for their capability to store CO 2 gas [11]. However, there has been little success with these materials due to their high hydrophilicity, like zeolites, or their low gas selectivity, like activated carbons [12]. Chemicals like polymers and biomass materials or activators are used to create active carbon media to enhance the efficiency of CO 2 gas capture 1 3 [13]. Another class of materials known as Metal-organic frameworks (MOFs) is investigated for gas capture. Basically, MOFs are made from inorganic node (metal ions) in addition to organic ligands. These materials are of increasing interest for green chemical engineering applications since it has a divers and unique structural properties, stability, ease of synthesis and design [14,15]. In contrast to traditional inorganic materials, MOFs can have precise control over composition, morphology, pore properties, and functionality [16]. The ease of manipulating the shape size of pores in MOFs with minor or rare chemical tuning making it attractive in improving the efficiency of the material for various applications [17,18]. In the quest for new adsorbents for CO 2 capture [41][42][43][44][45][46][47][48], we turned our attention to Schiff's base with metal complexes, because they are useful as biologically active compounds [19][20][21][22][23]. In particular, amoxicillin, which is used as an antibiotic to treat various infections, contains different aromatic rings and is characterized by high nitrogen and oxygen contents. Considering that these characteristics could be beneficial for CO 2 adsorption. This paper includes the synthesis of new amoxicillin-Schiff's bases complexes and investigated their application in CO 2 capture.

General
Amoxicillin (99.5%), P-bromobezaldehyde, and analytical grade metal chlorides complexes were supplied from Merck and used without further purifications. Sovents are also provided by Merck. The elemental composition of the complexes was determined by used Vario EL III elemental analyzer (Germany). The metal content was determined via an AA-6880 Shimadzu atomic absorption flame spectrophotometer (Japan). The ultraviolet-visible spectra were detected in ethanol via a Shimadzu UV-1601 spectrophotometer (Japan). The Fourier transform infrared spectra were measured by an FT-IR 8300 Shimadzu spectrophotometer (Japan). Corning conductivity meter 220 was used to measure the conductivity in ethanol at concentration of 10 -3 M and 25°. A Tescan MIRA3 LMU equipment (Czech Republic) was used to obtain Field emission scanning electron microscopy (FESEM) images.

Synthesis of Schiff's base ligand
A 1 mol (0.185 g) of p-bromo benzaldehyde and 1 mol (0.419 g) of amoxicillin trihydrate was dissolved and refluxed in ethanol for 6 h. The precipitate was filtered and washed off few times with ethanol to obtain the ligand. The product was dried under vacuum for 24 h.

Preparation of metal complexes
A 2 mol of 1 was dissolved in ethanol (10 mL) followed by the addition of metal ions (PdCl 2 or CoCl 2 .6H 2 O). The reaction mixture was then refluxed for 2 h. The resulted colored precipitate was collected and recrystallized from ethanol. The product was dried under vacuum for 24 h. The reaction was schematically presented in the Scheme 1.

Nitrogen gas adsorption measurements
All the samples were dried under vacuum for 4 h ate 60 °C before subjected to any evaluation. The specific surface area was evaluated by Brunauer-Emmett-Teller (BET) method using nitrogen (N 2 ↑) adsorption isotherms. The Barrett-Joyner-Halenda (BJH) method was used to measure the Pore size and pore volume [24].

Gas storage measurements
In order to eliminate any water and gas residuals with in the pores of the sample, the product (1 g) was set on a vacuum oven at 50 °C for 4 h prior to any measurements. H-sorb 2600 high-pressure volumetric adsorption appliance was used to measure the uptake of gas for the complexes. The experiment was reproduced several time under the same conditions to identify the best pressure work conditions as previously reported by L. Hauchhum et al. [25].

Synthesis and characterization of compounds (1) and metal complexes (2)
The main characteristics of compound 1 and metal complexes 2 (CoL 2 and PdL 2 ) are presented in Table 1. The obtained data are in a good agreement with calculated values. The product 2 (CoL 2 and PdL 2 ) have a crystalline and colored appearance with a satisfactory solubility in different solvents. The suggested structure was examined spectrally, magnetic moment and molar ratio.

FT-IR analysis
The structure of compounds 1 and 2 were examined and confirmed by FTIR spectroscopy and compared to values in literature [26,27]. Some significant absorption band of compound 1 at 1641 cm −1 due to the C=N stretching band confirming the formation of Schiff's base. In addition to a shifts in C=O of amide and C=O for lactam to 1680 and 1770 cm −1 respectively.

Electronic absorption spectra
The UV-vis spectrum of the dark green Co(II) complex, in the present work showed band at 15,290 cm −1 which due to 4 A 2 → 4 T 1 p. The 4 A 2 → 4 T 2 , and ν 2 4 A 2 → 4 T 1 transitions does not appear in the spectrum because it is out of the scale of UV device. These transitions are calculated from IR spectrum. It was found to be 3452 cm −1 . The second transition calculated theoretically from the Eq. 15B \ = v 3 + v 2 -3v 1 , using Tanabe-Sugano diagram for d 7 system. The results showed a transition at 5701 cm −1 . Furthermore, Tanabe-Sugano diagram d 7 system was used to calculate B' , Dq and β. In addition, absorption bands indicated at 26,737, 36,101 and 44,052 cm −1 are attributed to charge transfer from 1 to metals. The magnetic properties of Co(II) complex were measured and indicated at 4.85 B.M exhibiting a paramagnetic characteristic of high spin geometry [28,29]. The magnetic moment of Co(II) tetrahedral complex is higher than the spin only 3.873 B.M due to the contribution of coupling of the spin orbitals [30]. The calculated spin coupling constant λ' is compared to that of free ion. The estimated value of λ' for this Co(II) complex is -176 cm −1 which does not agree for range of the regular geometry. The value of conductivity of cobalt complex shows electrolyte state. From all these data tetrahedral structure can be proposing for this complex. The Pd(II) complex is diamagnetic and most of them are square planer. The analysis of UV-Vis spectrum of this complex that show the existence of three bands. The first one at 2624cm 1 and the second and third as a more intense bands at, 3024 and 36,101 cm −1 respectively, which might be assigned to the 1 A 1 g → 1 B 1 g, 1 A 1 g → 1 E 1 g and 1 → PdCT transitions respectively [31]. This came in accordance with published data for square planar palladium complex [32]. The conductivity measurements showed that the complex was ionic. From the analysis of data and spectroscopy techniques, a square planar around Pd(II) ion can be proposed.

1 H-NMR spectra
The synthesized compounds are characterized and confirmed via 1 HNMR in DMSO. A characteristic peak of 1 appears at 1-1.3 ppm are correlated to protons of ethyl group close to lactam ring in amoxicillin structure [33]. The singlet peak at 9.35 is attributed to azomethaine group in compound 1 [34] while the aromatic protons appears at 66-7.3 ppm as two doublets. The N-H protons exhibit a singlet peak at 8.5 ppm.

13 C-NMR spectra
The structure of 1 was also confirmed by 13 CNMR spectroscopy in DMSO. The assigned chemical shifts are attributed to lactam carbonyl carbon atoms at at δ 135. 31,131.67 and 129.91 ppm which explain to carbon atoms of C6, C7 and C8 of ß-lactam.

Morphology of metal complexes
The morphology of metal complexes were tested by FESEM technique. The FESEM images are illustrated in Figs. 1 and 2. The images revealed a heterogeneous and

Atomic force microscopy (AFM) of metal complexes
Surface morphology was studied using AFM. It provides a better understanding on the roughness and porosity of materials' surface of the adsorbent. This information is necessary in gas storage application. It provides a better understanding of the surface and cross-sectional image as well as the assessing the geometric feature of the 3-D AFM images for each particle. Surfaces exhibit a less roughness(Rq) have a smooth surface with less porous properties. Unlike higher Rq surfaces, the surface has high porous structure and less smooth surface. Therefore, Pd(II) complex reveals a higher Rq value, which means higher porous and rough surface. Figures 3 illustrates the AFM images of metal complexes.

The energy-dispersive X-ray (EDX) spectra of metal complexes
The energy-dispersive X-ray (EDX) spectra of metal complexes (Figures 4 and 5) revealed new bands corresponding to Pd, Co, nitrogen, and oxygen, in addition to carbon and chlorine atoms (high proportion). Figures 4 and 5 shows the abundance of elements in complexes.

BET surface area determination of metal complexes
The nitrogen (N 2 ) adsorption isotherm was analyzed using The Brunauer-Emmett-Teller (BET) method to calculate the specific area [35]. Figures 6 and 7 illustrates the N 2 adsorption isotherms in addition to pore size distribution throughout the material. The pore size and distribution provide the necessary information about the chemical and physical interaction of the adsorbed gas with adsorbent surface. The N2 adsorption isotherm was used to measure the BET surface and volume of the pores at P/Po = 0.9 Pore size and volume evaluated by BJH method. Surface area and pore size distribution (PZD) of metal complexes by the N 2 adsorption for CoL the SBET 6.63 m 2 .g −1 , Pore Volume 0.030 cm 3 .g −1 , Average pore diam-eter18.39 nm, 5.82Rq. And for PdL CoL the SBET 8.71 m 2 . g −1 , Pore Volume 0.0501 Cm 3 .g −1 , Average pore diam-eter22,98 nm, 12.61 Rq. The results revealed that PD(II) complexes have higher BET surface and pore diameter and volume.

Gas uptake of palladium complex
The gas uptake capability of the synthesized complex was identified via a high-pressure volumetric adsorption apparatus of type H-sorb 2600. The complex was degassed under vacuum in order to obtain accurate results, the gas uptake examination was replicated using identical conditions for the prepared samples to detect the optimal pressure. The adsorption of the gas depends on the pore size, metal charge, ligand type, and interaction strength between the adsorbate and adsorbent, for instance Van der Waal forces and H-bonding [36,[43][44][45][46][47]. The critical role of the pore volume is essential to determine the gas  uptake capacity of the adsorbent materials, where large pore volumes results in storing higher amounts of gases [37]. The quadrupole moment of CO 2 has a great effect on the adsorption capacity as it is highly diffusible [38]. In addition, strong attraction forces, such as electrostatic (polarization force and surface field-molecular dipole interactions) and Van der Waals forces, may also fulfill high CO 2 uptake [39-43, 48, 49]. The synthesized complexes were tested for CO 2 uptake and the results show an uptake of 21.38 and 26.16 cm 3 /g for Co(II) and Pd(II) complexes, respectively. Further investigation is recommended to employ this type of complexes for gas storage, which may contribute in finding a solution for the recent CO 2 high levels problem, and ultimately the global warming. The gas adsorption isotherms for complexes are illustrated in Fig. 8.

Conclusion
Herein, a palladium and cobalt complexes containing amoxicillin moiety was synthesized and characterized with different spectroscopic methods. The surface of complexes has particles that vary in shape, size, and diameter.
The results have shown that the complexes surface area is 6.63 and 8.71 m 2 /g, which makes it suitable for many applications. For gas storage application, the gas uptake depended on the nature of gas, structural metal, pore diameter, and the surface pore volume. Synthesized complexes were tested for CO 2 uptake and the results show an uptake of 21.38 and 26.16 cm 3 /g. Further investigation is recommended to employ this type of complexes for gas storage, which may contribute in finding a solution for the recent CO 2 high levels problem, and ultimately the global warming. The amoxicillin complexes derivative with -arrangement has the highest surface area and pore size distribution and serves as an efficient CO 2 storage material compared with its counterparts exhibiting Schiff's base compounds.
Funding No funding was received to assist with the preparation of this manuscript.

Conflicts of interest The authors declare no conflict of interest.
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