Thermal decomposition and kinetic analysis of a MIL-88A metal-organic framework

The thermal decomposition of the MIL-88A metal-organic frameworks (MOFs) is studied, for the first time, at various heating rates under air and Nitrogen. The precursors of a MIL-88A material revealed water evaporation and remaining solvent molecules in the framework structure at low-temperatures range while at the high-temperatures range, the organic skeleton collapse; in the meantime, Fe 0.94 O and the shell of graphitized carbon are formed. Thermogravimetric analysis (TGA) curves of MIL-88A moved slowly to the high-temperatures region with increasing heating rates. Additionally, Differential Scanning Calorimetry (DSC) curves of MIL-88A provided a change in the peak intensity, indicating the difference in the sample weight loss. X-ray diffraction (XRD) pattern and transmission electron microscopy (TEM) of the as-prepared sample confirmed the existence of a single hexagonal MIL-88A without any impurities of other phases. However, XRD exhibited two phases Fe 0.94 O and F 0 , after annealing. TEM of the annealed sample also asserted the creation of a carbon layer on the surface. The activation energy and kinetic parameter of the MIL-88A material offered higher values as the exothermic peak increased. Interestingly, the thermal decomposition of the MIL-88A material is effective in investigating the temperature effect on the pyrolysis process that can benefit in energy storage applications.


Introduction
Recently, the world is heading towards a green revolution to decrease pollution and carbon dioxide resulting from burning fossil fuels. Besides, our society increasingly needs high performance and safe energy storage devices [1]. Thus, the development of novel functional compounds with high performance is the critical step. One of these new functional materials is Metal-organic frameworks (MOFs).
MOFs represent unique porous compounds and containing organic linkers and metal ions [2]. The definite assemblage of organic linkers and metal ions produces a solid structure that spreads in different directions with great holes [2,3], low densities, and ultrahigh surface areas. According to these properties, MOFs have revealed various applications in greenhouse gas capture [4], drug delivery [5], energy storage [6], and cancer therapy [7].
Several properties of MOFs have been studied, such as the structure, electronic, electrochemical properties. However, the thermal characteristic and kinetic parameters of MOFs are still scarce. Therefore, the thermal decomposition of MOFs deserves interest to concoct for potential applications according to cruel provisions.
The thermal stability of MOFs dramatically depends on the strength of metal-ligand bonds and connectivity. Limited thermal stability is a major reason why MOFs are frequently outperformed by other compounds like MIL-88A in recent applications [8,9]. There are different types of MOF materials, such as UIO-66 (University of Oslo), MIL-88A (Materials Institute Lavoisier), and Zeolitic imidazolate frameworks (ZIFs). The MIL-88A material is a type of MOF that has some advantages, like higher porosity and great surface area than other MOFs. This study focuses on the MIL series, especially MIL-88A. The MIL series represents MOF structures that combine highly metal ions with carboxylate-based ligands, based on which many modified MOFs have been advanced by functional or extended bonds [10]. One of such series is a MIL-88A material with a three-dimensional supple framework based on Fe 3+ octahedral edges of the oxocentra axis connected by fumarate dihines, producing interrelated cages and pores with open canals operating along the c-axis [11]. Such an organization offers high porosity and a large specific area, rendering it appropriate for several applications such as carbon dioxide separation and absorption [12]. The MIL-88A material demonstrates a perfect content of coordinated unsaturated metal ions, which are the elementary reaction locations for host olefin molecules duringcomplex, rendering it suitable to absorb gases such as ethylene [13].
There are several papers offered on MIL-88A material. Amaro-Gahete et al. [14] synthesized highly crystalline MIL-88A particles with excellent properties. The effect of the synthesis time and sonication generator on the morphology, crystallinity, surface area, and structure of these materials were investigated. Liu et al. [15]   The powdered sample was contained in an alumina crucible and scanned at different heating rates (β = 5, 10, 15, 20, and 25 K/min). Linked data of pyrolysis process of the sample was obtained at a temperature ranging from 300 to 900 o K, with accuracy ±1K, under air and Nitrogen. The kinetics analysis was studied based on DSC data.
XRD data was collected via X-ray diffractometer Bruker D2 Phaser (CuKα, λ = 1.5417 Å) and documented in the 2θ range of 5-80 with 0.0101 steps. Consequent refinement was done by the Rietveld route in the FullProf package, as observed in the Ref. [18]. The morphology analyses were described utilizing transmission electron microscopy (TEM) (Tecnai G2 Spirit Bio TWIN) operating at 120 kV.

TGA and DSC
The pyrolysis process of the as-prepared MIL-88A metal-organic framework was tested by the thermogravimetric stability (TGA), which measured at a temperature ranging from 300 to 900 o K under Nitrogen and air. Figure 1(a) exhibits the TGA and DSC curves of the as-prepared MIL-88A metal-organic framework measured at a temperature ranging from 300 to 900 o K under air. It demonstrated that the precursors of MIL-88A suffered weight loss in stage A and additionally verified by exothermic of the process, as shown in Figure 1 (b), which may be referred to as water evaporation and remaining solvent molecules in the framework structure. As the temperature increased, the two steps of weight loss B and C stages were also detected due to the organic skeleton collapse; in the meantime, Fe0.94O and shell of graphitized carbon were formed [19]. This retention of carbon through the process of pyrolysis under air has been reported in other studies [20][21][22]. According to the further increasing temperature, the shells of carbon could be neglected from the yield product. The change in the peak intensity of DSC in Figure 1(b) refers to the difference in the sample weight loss. Based on TGA and DSC results (Fig. 1), the best temperature used to anneal the Fe0.94O coated by carbon shells at 723 o K. Figure 2(a,b) illustrates the TGA and DSC curves of the as-prepared MIL-88A metal-organic framework measured at a temperature ranging from 300 to 900 o K under N2 flow. There are principally three regions in a decomposition pattern, as shown in Figure 2(a). We noted a weight loss at around 373 o K due to water evaporation and the remaining solvent molecules at the first stage, also verified by the exothermicity of the process (Figure 2(b)). Furthermore, an organic ligand was gradually decomposed in a pair of stages, allowing the whole collapse of an organic structure at around 693°K [16,21]. It is observed from Figures 1(a) and 2(a) that TGA curves move slowly to the high-temperatures region as the heating rates increased. This is due to the aggravation of the temperature gradient between the samples and the surrounding areas at high heating rates. It is also clear from Figures 1(b) and 2(b) that the exothermic peak (Tp) in DSC curves moves to high temperatures as the heating rates increased, signifying the kinetic nature of the crystallization is present.
It is observed that the exothermic peaks obtained in DSC curves (Figures 1(b), and 2(b)) are broad, signifying the presence of more overlapped peaks. To separate overlapped peaks, the deconvolution process is applied using the Origin program with Gaussian-type function (Figure 3(a,b)). The deconvolution of overlapped exothermic peaks at heating rate β = 25 K/min is offered in Figure 3(a,b), as an example. The exothermic peaks overlapped may be due to phase separation that occurs upon the synthesizing process. The atmosphere of annealing affects the composition; hence in the case of annealing the materials under air, there are four stages from evaporating the water up to forming Fe0.94O, and each stage is represented by a peak, as demonstrated in Figure 3(a). However, we noted three stages under N2 annealing from water evaporation up to forming carbon layer on the Fe, as shown in Figure 3 Figure 4 shows the Rietveld refinement of the XRD pattern for MIL-88A. This refinement emphasized that the as-prepared composition is a single hexagonal MIL-88A with space group P-62c, lattice parameters a= 13.29Å, and c= 12.57Å, and there are no impurities by another crystalline phase. The observed data is a red line; the calculated pattern is a black line; the green line is Bragg reflection, and the blue line is the difference between the calculated and observed data. The XRD of the annealed sample is exhibited in Figure 4(b). It appears that all peaks are indexed to two phases Fe0.94O and Fe 0 , according to COD 2311020 and Pdf2-060696, respectively. To investigate the microstructure of MIL-88A materials after and before annealing, the measurements of TEM are utilized, as shown in Figure 5(a, b). The as-prepared MIL-88A contains microcrystals with a mean width of 1 μm and a smooth surface, as shown in Figure 5(a). However, after annealing under Ar at 723 o K, the precursor of MIL-88A converted from solid to nano assembled bipyramid with layers from carbon formed on the surface Figure 5(b) [19].

Kinetics analysis
To examine the crystallization kinetics of MOF materials, kinetic's parameters, such as activation energy, can be determined. The most important kinetic parameter to understand the crystallization process of MOF materials is the activation energy.
Where E represents the activation energy for crystallisation reaction, ko represents the factor of frequency, the R = gas constant. The crystallization of non-isothermal is described by the rate constant of heating. The activation energy of the MIL-88A can be determined by Kissinger's equation, which is obtained from Eq. (3), where is the heating rate, T is the maximum peak temperature, A is the frequency factor, and R is the universal gas constant (8.314 J/mol K) [25,26,27].
The temperature of max-peak for each exothermic reaction is affected by various  Table 1. It is observed that the activation energy and kinetic parameter of the MIL-88A under air and Nitrogen increase as the exothermic peak increased. This is due to the exothermic peak (Tp) shifting to high temperatures as the heating rates increased. On the other hand, the kinetic parameter and the activation energy of the MIL-88A material provided this behavior due to the nature of the decomposition mechanism in the process of pyrolysis. Interestingly, the thermodynamics analysis of the MIL-88A material is an effective way to explore the temperature influence on the process of pyrolysis, which can benefit in several recent applications. Remarkably, the thermal decomposition of the MIL-88A compound is an effective method for studying the temperature effect on the pyrolysis process that can be useful in energy storage applications.

Declaration of Interest Statement
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.