1 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, CO2 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 CO2 gas [7]. Ethanol amine process is proven to be a good adsorbent for CO2 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 CO2 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 CO2 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 CO2 gas capture [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 CO2 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 CO2 adsorption. This paper includes the synthesis of new amoxicillin-Schiff’s bases complexes and investigated their application in CO2 capture.

2 Experimental

2.1 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.

2.2 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.

2.3 Preparation of metal complexes

A 2 mol of 1 was dissolved in ethanol (10 mL) followed by the addition of metal ions (PdCl2 or CoCl2.6H2O). 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.

Scheme 1
scheme 1

Synthesis of metal complexes

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2.4 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 (N2↑) adsorption isotherms. The Barrett-Joyner-Halenda (BJH) method was used to measure the Pore size and pore volume [24].

2.5 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].

3 Result and discussion

3.1 Synthesis and characterization of compounds (1) and metal complexes (2)

The main characteristics of compound 1 and metal complexes 2 (CoL2 and PdL2) are presented in Table 1. The obtained data are in a good agreement with calculated values. The product 2 (CoL2 and PdL2) have a crystalline and colored appearance with a satisfactory solubility in different solvents. The suggested structure was examined spectrally, magnetic moment and molar ratio.

Table 1 The main characteristics of 1 and its complexes 2
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3.2 Spectral analysis

3.2.1 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. The complexation of 1 with Co(II) and Pd (II) resulted in shifting of C=N stretching band of 1 to a higher wave number of 1658 and 1666 cm−1 respectively. Also, the absorption band at 3211 cm−1 belongs to NH stretching of 1 was shifted to 3217 and 3383 cm−1 for Pd(II) and Co(II) coordination’s respectively. The appearance of new absorption band below 500 cm−1 attributed to the M–N linkage. All the absorption bands support the complexation of 1 with Pd or Co to form compound 2. The main FTIR absorption bands are illustrated in Table 2.

Table 2 Selected FTIR absorption bands of metal complexes
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3.3 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 4A2 → 4T1p. The 4A2 → 4T2, and ν2 4A2 → 4T1 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\ = v3 + v2-3v1, using Tanabe–Sugano diagram for d7 system. The results showed a transition at 5701 cm−1. Furthermore, Tanabe-Sugano diagram d7 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 2624cm1 and the second and third as a more intense bands at, 3024 and 36,101 cm−1 respectively, which might be assigned to the 1A1g → 1B1g, 1A1g → 1E1g 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.

3.4 1H-NMR spectra

The synthesized compounds are characterized and confirmed via 1HNMR 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.

3.5 13C-NMR spectra

The structure of 1 was also confirmed by 13CNMR 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.

3.6 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 porous structure with aggregate of different sizes and shapes. The size of the particles is varied from 100.5–138.9 and 50.24–229.7 nm for the Co(II), and Pd(II) complex, respectively.

Fig. 1
figure 1

FESEM image of Co(II) Complex (a: 10 \(\mu m\) and b: 200 nm)

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Fig. 2
figure 2

FESEM image of Pd(II) Complex (a: 10 \(\mu m\) and b: 200 nm)

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3.7 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.

Fig. 3
figure 3

AFM images for (A) Co(II) complex (B) Pd(II) complex

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3.8 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.

Fig. 4
figure 4

EDX image for PdL complex

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Fig. 5
figure 5

EDX image for CoL complex

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3.9 BET surface area determination of metal complexes

The nitrogen (N2) adsorption isotherm was analyzed using The Brunauer- Emmett- Teller (BET) method to calculate the specific area [35]. Figures 6 and 7 illustrates the N2 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.

Fig. 6
figure 6

N2 adsorbed isotherms of Co(II) complex

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Fig. 7
figure 7

N2 adsorbed isotherms of Pd(II) complex

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Surface area and pore size distribution (PZD) of metal complexes by the N2 adsorption for CoL the SBET 6.63 m2.g−1, Pore Volume 0.030 cm3.g−1, Average pore diameter18.39 nm, 5.82Rq. And for PdL CoL the SBET 8.71 m2.g−1, Pore Volume 0.0501 Cm3.g−1, Average pore diameter22,98 nm, 12.61 Rq. The results revealed that PD(II) complexes have higher BET surface and pore diameter and volume.

3.10 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 CO2 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 CO2 uptake [39,40,41,42,43, 48, 49]. The synthesized complexes were tested for CO2 uptake and the results show an uptake of 21.38 and 26.16 cm3/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 CO2 high levels problem, and ultimately the global warming. The gas adsorption isotherms for complexes are illustrated in Fig. 8.

Fig. 8
figure 8

Adsorption isotherms of CO2 gas on the Pd(II) and Co((II) surfaces

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4 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 m2/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 CO2 uptake and the results show an uptake of 21.38 and 26.16 cm3/g. Further investigation is recommended to employ this type of complexes for gas storage, which may contribute in finding a solution for the recent CO2 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 CO2 storage material compared with its counterparts exhibiting Schiff's base compounds.