Coordination polymers based on trans, trans-muconic acid: synthesis, structure, adsorption and thermal properties

Introduction

Over the past two decades coordination polymers (CPs) have become one of the most rapidly expanding class of compounds in the chemical science due to their easily tunable structural characteristics and potential applications in catalysis, separation, gas storage, drug delivery, luminescence, non-linear optics, electrical conductivity and magnetism [1], [2], [3], [4], [5], [6], [7]. Judicious choice of metal ions, multidentate organic ligands and the reaction conditions allows to rationally control of molecular organization in the solid-state of coordination polymers [8]. Special approach for the synthesis coordination polymers with repeating polynuclear cluster centers which are termed as secondary building units (SBUs) is being realized in reticular chemistry [9]. In contrast to the single-metal nodes known in coordination chemistry since Hofmann and Kuspert such building units are joined together by strong directional bonds and impart architectural and mechanical stability to the framework of coordination polymer [10]. In wide range of recent developments were showed that removal of coordinated solvent molecules from coordination polymer structure under vacuum/thermal activation in some cases resulting in materials having the permanent porosities and morphology similar to traditional porous solid materials [11], [12], [13], [14], [15]. Such subcategory of CPs in the scientific community are generally termed “metal-organic framework” as suggested by IUPAC [16]. The high internal surface area, tunable pore size by using linking ligands of different dimensions or changing the connectivity of the inorganic building block, high metal content with possible coordinatively unsaturated metal ion centers, adjustable surface group functionalities, the possibility of designing coordination polymers from more than one metal ions/clusters or using as template for immobilizing of additional catalytic metal center/ligands into structure, the existence of a vast variety of post-synthetic modifications of CPs are creating enormous commercial and research possibilities for the scientific community [17], [18], [19], [20].

Herein, we report the synthesis of coordination polymers based on trans,trans-muconic (H₂Muco) acid and transition metals (Cu, Zn, Ni, Co), post modification of the prepared Co, Cu, Zn-based coordination polymers using solid-phase thermolysis to carbon functional materials with stabilized metallic species. Trans,trans-muconic acid was taken as a long chain organic ligand; 4,4′-bipyridine (4,4′-bipy; BiPy) and 1,2-bis(4-pyridyl)- ethylene (EtDiPy) have been introduced in Ni and Co-containing coordination polymers as bidentate pillar linkers for the construction of the cross-linked chains and as N-containing precursors for preparation of the N-doped nanocomposites. To our knowledge, there are not reports in using coordination polymers based on trans,trans-muconic acid as precursors for a desirable type of nanoparticles. On the other hand, interest to conversion of metal-containing coordination compound to N-doped carbon materials incorporating metal nanoparticles or surface-oxidized metal oxide species is caused by a unique combination of properties, including high catalytic activity and possibility of formation of nanocomposites with a relatively high concentration of metal phase [21], [22]. Thus, we focused our attention to understanding the underlying structure-property factors of parent structures which affected on thermolysis product microstructure.

Experimental section

Results and discussion

Composition, morphology and microstructure of the obtained compounds were studied by elemental analysis, X-ray powder diffraction, scanning electron microscopy and IR spectroscopy. Based on the experimental results the empirical formulas of the obtained compounds have been proposed. Elemental and analytical analysis data are presented in Table 1.

All the vibration bands of muconic acid and 4,4′-bipyridine in IR spectra (Fig. 1) are in good agreement with the published data on Spectral Database [24]. The conjugation of the carbonyl group with multiple bonds reduces the value of the stretching vibration of C=O in muconic acid to 1676 cm⁻¹. The two bands at 1076 and 1404 cm⁻¹ are described as in-plane ring deformation and in-plane C–H bending, respectively, in the vibrational spectra of 4,4′-bipyridine [25].

Fig. 1:

Fourier transforms infrared spectra of trans,trans-muconic acid and 4,4′-bipyridine.

According to IR spectroscopy all coordination polymers show strong and broad absorption bands in the range 2800–3600 cm⁻¹, corresponding to the stretching vibration of OH group of both coordinated and lattice water molecules and to stretching vibration of CH group in muconic acid molecules. The stretching vibration of CH in the aromatic ring of the coordinated 4,4′-bpy and EtDiPy molecules also appear in this area. The stretching vibration of C=O carbonyl group at 1676 cm⁻¹ in ligand is significantly shifted to 1588, 1608, 1620 and 1625 cm⁻¹ in structures CoMucoBiPy (b)), CoMuco (a), NiMucoEtDiPy (e) and NiMucoEtDiPy (f), respectively (Fig. 2). The difference between ν_as (COO) and ν_s (COO) provides useful information about the nature of carboxylate coordination [26]. It is assumed that the carboxylate ion coordinate to a metal cation in monodentate mode of coordination (∆ν≥203 cm⁻¹). The C=N and C=C stretching vibration band of the ligands appeared in the range of 1500–1625 cm⁻¹. Metal-oxygen and metal-nitrogen bonding was manifested by the appearance of a band in the 549–562 and 372–420 cm⁻¹ region, respectively.

Fig. 2:

Fourier transforms infrared spectra of coordination polymers (CoMuco (a), CoMucoBiPy (b), NiMucoEtDiPy (e), ZnMuco (h)).

The thermal stability of the obtained coordination polymers and trans,trans-muconic acid was investigated by thermogravimetric analysis (TGA-MS) performed simultaneously with DSC. Mass spectrometer was integrated with to the off-gas port of a NETZSCH system to analyze the gases that leave the material as it is heated. According to thermogravimetric data the decomposition of trans,trans-muconic acid is observed at 310.7°C. Decomposition temperature is taken as the peak temperature of the first derivate of the temperature with time (Fig. 3).

Fig. 3:

Thermal analysis curves (TGA/DSC) and mass spectra of gaseous products of trans,trans-muconic acid (heating rate 5° min⁻¹, under argon atmosphere over the temperature range 28–500°C).

Thermal analysis has proved to be useful in determining the crystal water content in the coordination polymers and their thermal stability as well as decomposition mode under controlled heating rate. TGA (a) and DSC (b) curves are displayed in Fig. 4. Their thermal decomposition data are tabulated in Table 2. In general, the initial continuous weight loss up to 95°C was observed for dehydration of guest solvent molecules. The second step was found to be in temperature range 105–207°C associated with the loss of coordinated water molecules. Decarboxylation was identified to be in temperature more than 160°C followed by the release of fragments of the backbone. Nickel(II) and copper(II) muconates exhibit lower thermal stability of the framework as compared with Co(II) muconate. The addition of bidentate pillar linker in the structure of Co(II) muconate reduces thermal stability of the initial coordination polymer.

Fig. 4:

TGA (a) and DSC (b) curves of coordination polymers based on trans,trans-muconic acid and transition metals (Cu, Ni, Co), (heating rate 5° min⁻¹, under argon atmosphere over the temperature range 28–500°C).

The morphology, phase identification and crystallinity of coordination polymers based on divalent transition metal and the products of thermolysis were characterized by scanning electron microscopy and X-ray diffraction (Fig. 5) techniques. Addition information about the elemental composition of the products of thermolysis was obtained by elemental analysis (Table 3).

Fig. 5:

PXRD patterns of coordination polymers (CoMucoBiPy (b), ZnMuco (h), CuMuco (g)) and the thermolysis products (CoMucoBiPy therm, ZnMuco therm, CuMuco therm).

The X-ray diffractogram showed amorphous peaks with a crystalline fraction. The CuMuco decomposed to crystalline copper nanoparticle (JCPDS No. 4-836) and a copper oxide (JCPDS No. 5-667) during the heating process under vacuum. The diffractogram of ZnMuco demonstrated peaks correspond to ZnO nanoparticles (JCPDS No. 79-2205). Thermolysis product of CoMucoBiPy contain Co nanoparticles (JCPDS No. 15-806) and CoO (JCPDS No. 75-419). Moreover, the compositions of the carbon matrix and the metal oxide nanoparticles were confirmed by energy dispersive X-ray spectroscopy (EDS) analysis (Fig. 6).

Fig. 6:

Energy-dispersive X-ray spectrum of metallic cobalt nanoparticle and CoO that is prepared from CoMucoBiPy.

According to SEM images (Fig. 7) ZnMuco therm and CoMucoBiPy therm have a similar shape and size as the parent ZnMuco and CoMucoBiPy, respectively, which indicates that the morphology of initial materials was maintained during thermolysis. Product of thermolysis of CuMuco are spherical in shape with a relatively high concentration of metal phase according to elemental analysis data (71.5%, Table 3), unlike the initial coordination compound. Two small exothermal peaks around 279.8°C, 380.2°C in DSC (Fig. 4b) curve of copper muconate indicated that copper nanoparticles formed during the heating process probably catalyze the decomposition of the sample. But this aspect requires further detailed investigation.

Fig. 7:

SEM images of the thermolysis products (b–f) and starting coordination compound (a–e); CuMuco (a, b); ZnMuco (c, d); CoMucoBiPy (e, f).

The average size of nanoparticles in the product of thermolysis of copper muconate was calculated using the Scherrer equation: d=Kλ⁄(β cos θ), where d is the mean size of domains (crystallites), K is the dimensionless shape factor (the Scherrer constant) which usually is set equal to 0.9, β is the width of a diffraction peak (on half of the maximum intensity), λ is the X-ray wavelength and θ is the Bragg angle. The results showed that the average size of copper nanoparticles in CuMuco therm is about 20–22 nm. The average size of nanoparticles in the product of thermolysis ZnMuco and CoMucoBiPy in SEM was estimated using Image J software. According to data received, the composite prepared by the thermolysis of ZnMuco consists of rod-shaped nanoparticles with diameters about 45–55 nm. Formation of nanosheets with width of 70–80 nm is observed for CoMucoBiPy.

The N₂ adsorption–desorption isotherms of the samples measured at 77 K after evacuation and the pore size distribution curves are shown in Figs. 8–11. BET surface area and porosity properties of samples are listed in Table 4. Nitrogen adsorption isotherms of coordination compounds are of type III according to the IUPAC classification [27] and therefore no identifiable monolayer formation.

Fig. 8:

N₂ adsorption-desorption isotherm and the pore size distribution curve for NiMucoEtDiPy (e) dried in vacuum at 60°C.

Fig. 9:

N₂ adsorption-desorption isotherm and the pore size distribution curve for NiMucoEtDiPy prepared by solvothermal synthesis and dried in vacuum at 60°C (f)* and for NiMucoEtDiPy (e)# prepared by precipitation and dried in vacuum at 95°C.

Fig. 10:

N₂ adsorption-desorption isotherm and the pore size distribution curve for CoMucoBiPy (b) and CoMuco (a).

Fig. 11:

N₂ adsorption-desorption isotherm and the pore size distribution curve for CuMuco (g), ZnMuco (h), NiMuco (c).

There is a hysteresis loop between the adsorption and desorption for NiMucoEtDiPy and CuMuco (Figs. 8 and 11). Such an adsorption behavior of nitrogen is typical for an adsorption process in which a guest-induced phase transition takes place [28].

Method of synthesis of coordination polymers and drying temperature significantly effect on the values of specific surface area and pore size, as well as the nature of their distribution (Figs. 8 and 9, Table 4). The biggest value of the specific surface has a compound e, obtained by reacting a metal salt with muconic acid in the presence of a bidentate pillar linker – 1,2-bis(4-pyridyl)-ethylene at room temperature and atmospheric pressure in water. Obtaining the same compound in solvothermal conditions reduces surface area and the volume of pores is noticeable decreased (for f*). Practically the same effect is exerted by the additional heat treatment of this compound (for e#). In all cases addition of bidentate pillar linker increases surface area of coordination polymers.

Conclusions

In summary, series of coordination polymers based on trans,trans-muconic acid and transition metals were obtained and characterized using a number of analytical techniques including powder X-ray diffraction, elemental analysis, thermal gravimetric analysis, scanning electron microscopy and volumetric nitrogen adsorption/desorption. We have for the first time employed coordination polymers based on muconic acid as a template to prepare materials consisting of nanoparticles stabilized in a carbon matrix through solid-state thermolysis under vacuum. It should be noted that the morphology of initial materials was maintained during thermolysis, therefore the appropriate choice of metal-organic precursors in solid-phase thermolysis allowed to obtain various forms of carbon nanomaterials ranging from 0D to 3D. This study demonstrated that it is possible to design of N-doped carbon nanomaterials from 4,4′-bipyridine containing coordination polymers.