Amino-grafting pre-functionalization of terephthalic acid by impulse dielectric-barrier discharge (DBD) plasma for amino-based Metal-Organic Frameworks (MOFs)
Graphical abstract
Introduction
Considered as among the most promising novel materials because of their wide versatility, Metal-Organic Frameworks (MOFs), also known as porous coordination polymers are porous hybrid compounds formed by self-assembly of metal clusters (secondary building units (SBU)) with polytopic organic ligands to form crystalline networks [[1], [2], [3]]. MOFs exhibit great diversity in terms of structure and composition. Therefore, their intrinsic nature gives them impressive features, in particular high porosity with a wide range of pore size and high internal surface area [4]. These traits make them attractive across a wide range of applications, such as gas storage and separation [[5], [6], [7]], catalysis [[8], [9], [10]], sensing [[11], [12], [13]], optics [[14], [15], [16]], batteries [[17], [18], [19]], among many other fields.
In addition to the wide variety of combinations of organic and inorganic entities used to design MOFs with desired topologies and characteristics, their properties can also be modulated or even enhanced by tuning the surface chemistry via functionalization process. Usually, the functionalization process of MOFs is carried out via wet-chemical methods [[20], [21], [22]]. There are two main routes to prepare functionalized MOFs:
- (i)
pre-functionalization that involves the addition of functional groups into the phenyl ring of the organic ligand. This effective tool was initially developed by the group of O. Yaghi [23] who synthesized a series of isoreticular MOFs (IRMOFs) by grafting some functional groups, such as -Br, –NH2, –OC3H7, OC5H11 in the organic ligand (terephthalic acid) of MOF-5. In this way, other series of MOFs with different organic functionalities and isostructural topologies were synthesized, like the series of MIL-53-Xn (Xn = Cl, Br, NH2) of Férey et al. [24].
- (ii)
Post-synthetic modification of MOFs, initially suggested by Hoskins and Robson [25], which occurs by chemical modification of the network after the synthesis [[26], [27], [28]]. The main strategies for functionalization of MOFs by post-synthetic modification include covalent, dative modifications and those by encapsulation [[29], [30], [31]]. The scope of this paper relates to the pre-functionalization of the organic ligand.
In the literature, several computational and experimental studies of MOFs’ functionalization report on the effectiveness of introducing specific substituent groups on the ligand structure. It was shown that grafting functional groups, such as amino groups on the aromatic ligand, enhances the selectivity of MOFs toward certain gases, and improves the interaction between the adsorbent and adsorbate, particularly for CO2 and H2 [[32], [33], [34]]. Therefore, many works have been carried out to improve the CO2 uptake on MOFs. Rada et al. [35] studied the CO2 adsorption on amino functionalized titanium-based MOFs, NH2-MIL-125 (Ti), and compared it with the pristine material MIL-125 (Ti). These authors pointed out that the addition of amino functional group increases remarkably the CO2 uptake and affinity, with an adsorption capacity of 10.76 mmol/g and 8.9 mmol/g at 273 K and 298 K, respectively, much higher than those of untreated MIL-125 (Ti) (4 mmol/g and 2,8 mmol/g at 273 K and 298 K, respectively). As for H2 uptake, Klopper et al. [36] showed that grafting an electron-donating radical such as amino into the phenyl ring of the organic ligand improves the energy of interaction of H2 with the phenyl ring. Wu et al. [34] studied sorption enthalpy of MIL-68 (In) and an amino functionalized MIL-68 (In). They showed that the amino group in NH2-MIL-68 (In) could indeed improve the adsorption kinetics of H2 on the material. Besides gas storage/selectivity improvements of some MOFs by amino functionalization, this latter can also enhance their tailorability toward drug/gene delivery [[37], [38], [39], [40]], asymmetric catalysis [[41], [42], [43]], tissue engineering [[44], [45], [46]], etc.
Besides the wet chemical methods, a third possible route to functionalize MOFs consists in using low temperature plasma treatments. Plasma treatments are widely used to functionalize different materials like polymers [[47], [48], [49]], cotton [[50], [51], [52]], wood [[53], [54], [55]], glass [[56], [57], [58]], metals [[59], [60], [61]]. The dry plasma processes used for the elaboration and functionalization of materials are varied. In fact, they differ according to the gas pressure, ranging from low pressure processes (a few Pa to 100 Pa) [48,54,[62], [63], [64]] to atmospheric pressure processes [49,51,[55], [56], [57],65]. Low-pressure plasma processes are highly non-equilibrium thermodynamics means with high electron temperatures that can reach a few tens of electron-volts and a low degree of ionization [66]. On the other hand, plasmas at atmospheric pressure can have a high degree of ionization but low electron temperatures [67]. Therefore, the pressure difference from one process to another plays a major role in the physical and chemical actions the plasma will have on the targeted material. Indeed, plasma processes provide treatments at non-equilibrium thermodynamic state, thus allowing novel reaction possibilities. The assets of these cold plasma treatments are based on factors such as:
- (i)
The bombardment of the surface of a material by energetic species of the plasma, which generates a breaking of some covalent bonds and the formation of free radicals. These radicals react with the active species of the plasma, which results in the formation of functional chemical groups on the surface of the materials depending on the nature of the gas phase.
- (ii)
Plasma treatments can be carried out in dry conditions, thus reducing the use of chemicals and solvents, and therefore the energy to recycle or eliminate the residues.
- (iii)
Consequently, from a security point of view, there is less vulnerability to chemical risks with plasma treatments, compared to the wet chemical methods.
In this paper we report on a novel pre-functionalization process by a dry chemical method using a dielectric barrier discharge (DBD) plasma treatment of terephthalic acid (C6H8O4). This acid is known as the major organic ligand used for MOFs' synthesis due to its rigidity, its variety of architectures, and its versatile coordination modes. Although there are many studies focusing on the MOFs’ functionalization, none of these studies concerns the plasma functionalization. Therefore, the scope of this study is to examine and evaluate the efficiency of the DBD plasma treatment to realize a pre-functionalization by grafting amino groups on the organic ligand. The idea is first to have a proof of concept of the feasibility of the dry chemical pre-functionalization via DBD treatment on the organic ligand, and second to evaluate the efficiency of the DBD plasma treatment for MOFs functionalization. Therefore, plasma diagnostics and material characterization are carried out to examine the evolution of the plasma discharge during the treatment, to determine the chemical species present in the plasma, and to check the efficiency of the grafting process.
Section snippets
DBD setup and material
A schematic diagram of the experimental setup is illustrated in Fig. 1. The DBD system is set up in a typical planar symmetric configuration of two dielectric plates of square shape placed opposite to each other. A copper electrode is placed on the backside of each dielectric plate. The upper square electrode of 10 cm2 is connected to the high voltage while the lower electrode with same external dimension is grounded (electric reference potential of the system). The lower electrode has a
Voltage-current characteristics
Fig. 2 shows the time evolution of the applied voltage and the discharge current during the DBD treatment. A current measurement without the powder (equivalent to t = 0 min) is also presented in the figure to compare the powder effect on the discharge current. Based on the voltage curve, a value of 2 kV of voltage is sufficient to ignite the plasma. As for the discharge current, its maximum peak current has a duration Δt1 = 75 ns. Moreover, the maximum of the current intensity increases
Conclusion
In this work, an amino pre-functionalization study of terephthalic acid ligand by a NH3 DBD plasma treatment are investigated. The different diagnostic and characterization techniques performed in the present study demonstrate the effectiveness of the DBD plasma treatment to graft amino groups into the organic ligand. The plasma diagnostics reveal the short duration of the DBD discharge, the stability of the plasma treatment over the process time, and the presence of nitrogen-containing
CRediT authorship contribution statement
Aymane Najah: Formal analysis, Conceptualization, Writing – review & editing, Writing – original draft, Data curation, Investigation. Dimitri Boivin: Writing – review & editing. Cédric Noël: Validation, Supervision, Resources. Ludovic De Poucques: Validation, Supervision, Resources. Gérard Henrion: Validation, Supervision, Writing – review & editing. Stéphane Cuynet: Validation, Supervision, Writing – review & editing.
Declaration of competing interest
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.
Acknowledgements
This work has been funded by the french Agence National de la Recherche (ANR) in the frame of the general launch call 2020 with the projet named SYNERGY <ANR-20-CE05-0013>.
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