Chapter 1
The Stability of Metal-Organic Frameworks

Georges Mouchaham1, Sujing Wang2 and Christian Serre2

1Université Paris Saclay, Université de Versailles St-Quentin en Yvelines, Institut Lavoisier, UMR CNRS 8180, 45 Avenue des Etats-Unis, 78035, Versailles Cedex, France

2 PSL Research University, Institut des Matériaux Poreux de Paris, FRE 2000 CNRS, Ecole Normale Supérieure, Ecole Supérieure de Physique et des Chimie Industrielles de Paris, 75005, Paris, France

1.1 Introduction

Metal-organic frameworks (MOFs), also known as porous coordination polymers (PCPs), are a recent class of hybrid materials that have attracted considerable attention during the last few decades. Their crystalline structure constructed from the association of inorganic building units connected through complexing organic linkers can be wisely and finely tuned [1, 2], in terms of the chemical nature (metal cations, organic linkers), pore size (micro- or mesoporous), and the shape/type of cavities (cages or channels and triangular, square, or hexagonal, etc.). As a consequence, the versatile and tunable characteristics of MOFs have, nowadays, made them very promising candidates for various applications including gas storage, molecular separation, biomedicine, sensing, catalysis, and so on [3, 4].

Because the particularity of each MOF, for a given property or application, derives from its unique well-defined crystalline chemical structure, its porous framework shall be expected (and mandatory) to be retained without any alteration over the course of a process. In other words, for practical applications, one shall carefully consider the stability as one of the most important requirements to be fulfilled.

In a general manner, and particularly in the field of MOFs, "robustness" or "stability" cannot be considered as an absolute qualification, and one shall consider a predefined set of parameters. These depend on the targeted application where the porous material will be exposed to a given environment (i.e., organic solvents, water, corrosive media, etc.; high temperature and/or compression), at a certain concentration and for a given duration. Consequently, depending on the considered criteria, three main categories of stabilities can be identified: (i) chemical stability, (ii) thermal stability, and (iii) mechanical stability, where, henceforth, stability of a MOF refers to the resistance of its structure to degradation upon exposure to the operating conditions. Though, it is worthy of note that thermal stability (except in the case of amorphization and melting; see Section 1.3) is closely related to chemical stability because heating may also alter the chemical structure of the MOFs by initiating and/or accelerating the chemical reaction inducing the degradation of the corresponding crystalline framework. This affects mainly the coordination sphere of the metal cation due to the disruption of the coordination bonding between the organic ligand and the inorganic moiety (i.e., hydrolysis, redox activity, etc.), or, sometimes, it can affect the organic linker itself (i.e., decarboxylation, alkyne oxidation, etc.).

Regarding an application of interest, an MOF shall possess one or more types of stabilities. For example, chemical stability is crucial for applications in aqueous media and/or at different pH, such as in molecular separation or drug delivery [5], while both chemical and thermal stabilities are important for catalytic processes performed under harsh conditions as for chemical feedstock and fuel production [6]. Mechanical stability is mainly considered in MOF shaping such as making pellets or other compact forms required in industrial processes [7].

If, at the early stage of MOF exploration, one of the main concerns was to synthesize frameworks possessing the highest surface area and the largest pore volume, more recently considerable efforts have been devoted to the design of highly stable structures, allowing applications under ambient conditions as well as in harsh and corrosive media. This chapter aims at giving a comprehensive overview of the three aforementioned categories of stabilities and their importance in MOFs. Particular attention will be paid to address strategies allowing the synthesis of robust MOFs.

1.2 Chemical Stability

While thousands of different structures of MOFs have been reported to date, relatively, only a limited number have exhibited promising properties under non inert conditions (i.e., outside their mother liquor), in which they can be manipulated without alteration of their porous framework. MOFs consist of divalent cations (M2+) and carboxylate-based linkers are typical fragile materials [8, 9]. For instance, Zn2+ terephthalate MOF-5 degrades rapidly in water [8, 10-13], while Cu2+ trimesate HKUST-1 degrades over time in water at room temperature [14, 15]. The lack of stability for water is clearly a strong limitation on the use of MOFs not only for practical applications requiring a direct contact with water (e.g., separation processes from flue gas, which may contain considerable amounts of water, or water splitting catalysis [16, 17]), but also for clean applications such as hydrogen storage for fuel cells [18] in which water is itself a product of the reaction or could be a contaminant during the refueling process, for instance. In this context, chemical stability, in general, is one of the most basic criteria one should take into account in order to synthesize an MOF that is resistant to the ambient atmosphere in which water or moisture may be a considerable risk of degradation.

In 2009, Low et al. [9] have investigated, through a dual computational and experimental study, the effect of water (and steam) on a series of MOFs. They have highlighted the fact that the probability of the hydrolysis of the metal-ligand bond (involving breaking of coordination bonds and displacement of water (or hydroxide), ligated cations, and ligands) is inversely correlated with the strength of the bond between the metal cation and the organic linker from which the MOF is constructed. To some extent, this correlation can also be applied to all molecules (i.e., phosphate, H2S, SOx , NOx , NH3, phenolate, etc.) that can potentially compete with the organic linker and break the cation-ligand bond. Hence, regarding the potential competing agent (or reactive species), chemical stability can be subdivided into different categories such as moisture and water stability, stability to acidic or basic media, stability under harsh conditions (e.g., physiological media, in the presence of H2S or NH3, etc.), and so on. Accordingly, in order to improve the chemical stability of MOFs, efforts should be devoted to strengthening the interaction between inorganic and organic moieties. An alternative strategy to improve stability may be by preventing or limiting the access of any competing agent to the cation-ligand bond. A series of subfactors that should be considered, such as the redox behavior, the coordination geometry of the cation, the nuclearity and the connectivity of the inorganic building unit, the rigidity of the linker, the presence of open metal sites or defects, the hydrophobic character of the structure, framework catenation, inter- and intramolecular interaction, and so on, are also important to evaluate chemical stability.

Assessing the chemical stability of an MOF is most of the time performed by simply comparing the powder X-ray diffraction (PXRD) patterns of the sample before and after exposing the solid to a given environment. Nevertheless, this method does not give a complete and realistic evaluation of the stability even if the two patterns closely match. Partial degradation can still occur since such PXRD analysis is not quantitative and does not consider what could be released (solution) or the formation of amorphous phases. A complementary and more accurate analysis can be carried out by performing additional inert gas sorption isotherm measurements before and after the treatment. A loss of porosity will clearly reflect any partial degradation of the crystalline framework. Though, no standard methods of chemical stability tests (concentration, exposure time, number of cycles, etc.) or assessments have been established to date. Besides, when evaluating the stability as a function of pH, in most of the cases, verification of the pH of the solution after the addition of the MOF is not performed. Though, in the case of carboxylate-based MOFs, for instance, reporting the chemical stability at high pH values is in most cases inaccurate. In fact, partial degradation of the MOF is likely to occur, inducing partial release of the linker in solution, which in turn leads to a significant decrease in the initial pH of the medium. In addition, most authors indeed do not provide any analysis of the solution or the final solid to prove their assessments (e.g., BET surface measurements, thermal gravimetric analysis, etc.). For these reasons, in the following section, no accurate comparison of the stability of MOFs reported in different studies can be given. Examples discussed in this chapter will only rely on the self-statements of the authors.

The remainder of this section will encompass, in a non-exhaustive manner, the different methods that have been developed in order to enhance the chemical stability of MOFs. Because water stability is a very good indicator of a general chemical stability and a determinant factor for real-world applications, this section will be...