Nanoporous Solids: How Do They Form? An In Situ Approach

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Nanoporous solids: how do they form ? An in situ approach. Gérard Férey, Mohamed Haouas, Thierry Loiseau, and Francis Taulelle Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm4019875 • Publication Date (Web): 10 Sep 2013 Downloaded from http://pubs.acs.org on September 14, 2013

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Chemistry of Materials

Nanoporous solids: how do they form ? An in situ approach. ‡

Gérard Férey, *,† Mohamed Haouas, § Thierry Loiseau,† Francis Taulelle,§

Institut Lavoisier de Versailles (UMR CNRS 8180), Tectospin Group & Porous Solids Group, Université de Versailles Saint Quentin en Yvelines, 45, avenue des Etats-Unis, 78035 Versailles, France •

Corresponding author.

E-mail address: [email protected] (G. Férey) Web site : http://www.gerard-ferey.org/

RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to) Running title : Nanoporous solids : how do they form ? An in situ approach.

†

Porous Solids Group

§

Tectospin Group

‡

Present address : Unité de Catalyse et Chimie du Solide, UMR CNRS 8181, Université de Lille

Nord de France, 59652 Villeneuve d’Ascq, France

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ABSTRACT: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Understanding the crystallization mechanisms of nanoporous solids remains one of the most challenging issues in materials science. This Short Review focuses on the use of in situ nuclear magnetic resonance (NMR) spectroscopy under hydrothermal conditions to investigate the structure, the dynamics, and the stability/reactivity of the soluble species present in the synthesis medium during the crystallization. We describe how the formal SBU (Secondary Building Units) concept for solid construction can experimentally be investigated, checked and validated on some representative purely inorganic porous phosphates as well as hybrid metal organic framework (MOF) materials. We also discuss the specific role of reactive species identified in solution to lead to intermediate or more elaborate structures such as the PNBU (PreNucleation Building Units) or the MBU (neutral Molecular Building Units) respectively. In certain cases the proposed models could not to be generalized depending on the reaction conditions, the chemistry of the metallic cation, and the stability/solubility of the target phase. We also point out that experimental and theoretical approaches for identification and enumeration of existing and new SBU are essential for discovery and/or structure determination of new materials.

KEYWORDS: ULM & MIL compounds, nucleation/growth process, in situ XRD, Al-based coordination polymers, 27Al NMR.

1. Introduction

Since their discovery in 1756, with mineral stilbite, by the Swedish mineralogist Axel F. Crönstedt,1 porous solids, either natural or, later, synthetic, have known an increasing interest for scientists, up to becoming strategic materials. According to some authors,2 they represent more than 20% of the Gross Domestic Product of the industrial countries for the jobs, direct or indirect, that they now generate. Indeed, their numerous applications, primitively devoted to petroleum chemistry,


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catalysis, gas adsorption and storage reach now domains related to energy, energy savings, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

environment and health.3-5 These solids are alone to present three unique characteristics: a 3D framework corresponding to the tight binding of atoms within which periodically appear cavities (cages and/or tunnels) giving rise to internal surfaces at the frontier of matter and void. Chemically, the frameworks are of two types: purely inorganic (metallic silicates, aluminates, phosphates, arsenates…)6-9 or hybrid,10-12 in which the network is built up from both inorganic and organic parts linked exclusively by strong bonds. Depending on the conditions of synthesis, the inorganic part consists of either metallic ions, clusters, chains, layers and even their 3D arrangements. The organic linkers are mainly polycarboxylates, phosphonates, sulfonates or azolates. The size φ of the pores determines three subclasses for these solids: nanoporous (φ < 20 Å), mesoporous (20 Å < φ < 500 Å) and macroporous (φ > 500 Å). Most of the two latter are structurally amorphous. Whatever the type of porous solids, their chemistry is extremely rich,13 and led to the discovery and description of thousands of them. They usually were obtained using solvothermal synthesis in the range 120-250°C under autogenous pressure, using either the classical trials and errors method or, later, identified by high throughput techniques.14-16 However, concerning their formation and despite several hypotheses claimed without any support of experience, their mechanisms of association for providing the desired porous solid were completely unknown. It is however necessary if one wants to reach real « tailor-made » synthesis. The problem is that, during the hydro(solvo)thermal syntheses, the autoclave acts as a black box between the input of precursors and the output of products. The understanding of their mechanisms of formation requires to open it, and therefore, to use several complementary in situ techniques in real time,17,18 for identifying within the solution or solid-liquid interface the reactive species which lead to the final product. The choice of the techniques depends on the chemical nature of the atoms within these species and their sensitivity to the technique (i.e. NMR). This study has not, of course, the ambition to be general and is limited to some metallophosphates and porous carboxylates as representative examples for zeotypes and MOFs respectively. Silicate and aluminosilicate zeolites have been extensively investigated,19-26 and are beyond the scope of this 3 ACS Paragon Plus Environment


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review. At least, it describes some trends, which occur from the dissolution of the precursors in the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

solvent up to the precipitation of the final solids. These trends however open the way for the prediction of possible new porous topologies, rationally reachable.

2. A starting general idea

For a crystal chemist, the structures of porous solids can be described either from the classical connection of single polyhedra27 or, more recently, by the association of polynuclear units called SBU (for Secondary Building Units) whose translations, in the three directions of the space, generate the arrangements in the solid at the macroscopic scale.28 This latter approach has not only the advantage to better memorize the crystal chemistry of numerous solids, but led to the concepts of ‘scale chemistry’29,30 and ‘augmented nets’,31,32 which demonstrated the invariance of a given topology whatever the nature and nuclearity of the SBU. However, even if it became important later for the interpretation of mechanisms, it remained at that time just a tool of a posteriori description for the structures. An example is given in Figure 1.

Figure 1. Description of the structures of porous solids with (left) purely inorganic or (right) hybrid skeletons. The examples shown are ULM-5 or Ga16(PO4)14(HPO4)14F7(OH)2·[H3N(CH2)3NH3]4·6H20, and MIL-59 or (V(H2O))3O(O2CC6H4CO2)3·Cl·9H2O for inorganic and hybrid lattice materials respectively. ULM = Université Le Mans, and MIL = Materials of the Institute Lavoisier. ACS Paragon Plus Environment

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Thinking in terms of formation of porous solids, we proposed the following and rather naive hypothesis: the SBU that one observes in the final structure already exist as free species in the solution, before the precipitation which corresponds to the infinite condensation of these isolated SBU. One thing was to imagine that another was to prove (or eliminate) this idea… We undertook this research successively on the two types of microporous solids: first, those with a pure inorganic skeleton and, more recently, those with a hybrid organic-inorganic framework. Due to the different conditions of synthesis in both classes, the latter did not correspond to an extension of the first, but required a specific study. Indeed, the synthesis of inorganic porous solids requires the presence of templates (generally amines), which play an important role during the formation. For hybrids, the template is just the solvent; it has only a filling role. In our approach, our interest was focused only on the formation of porous solids in a pure aqueous medium. Moreover, the selection of the products submitted to our investigations was dictated by the techniques we chose for opening the black box. Among them, ex and in situ NMR took an important place. For this reason, we selected various porous metal phosphates (Al, Ga, Ti) synthesized by our group as representants of the inorganic class, and Al carboxylates for hybrids. They all contain various NMR nuclei.

3. Experiments and preliminary requirements

In situ NMR techniques were used for looking in real time and under various conditions (T, P and t) the evolution of the reaction, from the introduction of the precursors to the precipitation of the final solid and were developed in the nineties. We therefore designed a new set-up (Figure 2) allowing such observations33-37 fulfilling seven severe requirements. They must withstand (i) up to


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200°C, with a safety margin of up to 250°C, (ii) pressures up to 50 bars, (safety margin up to 200 bars), (iii) they must not react with reagents (HF, H3PO4, Al(OH)3, organic ligands and amines of different natures as well as any combination of these), (iv) they must resist to chemical aggressiveness of very basic as well as very acidic conditions (13 > pH > 1), (v) they must be almost transparent to radio-frequency, (vi) they must also have diamagnetic properties for avoiding a severe perturbation of the homogeneity of the B0 field (11.7 T for high resolution), (vii) no aluminum probe-head signal is desirable. This led to a probe modification.34 After calibration with several known solids containing Al, Ga, P, F, C, H and N nuclei, the first experiment consisted in the measurement of the true pH of the solutions in hydrothermal conditions by an original method (neutral pH = 6 at 150°C38).

Figure 2. (Left) Picture of the NMR tube and its components; (right) Hydrothermal NMR Tube design. (a) Teflon stopper; (b) titanium; (c) polyimide (Vesper Torlon); (d) volume restrictor (Teflon); (e) Teflon sleeve; (f) sample volume.

This new experiment has the unique advantage, during the whole reaction, to provide, beside the value of the true pH at 200°C, the identification and the quantification of the different species, vs. P, T and time, and their temporal evolution in the liquid. These in situ NMR investigations of the solution part constitute a complement to ex situ solid state NMR39-42 and in situ XRD19,43-45 studies of the solid part.


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4. Formation of porous metal phosphates

Three solids, obtained by hydrothermal synthesis at 180°C, were studied: (NH4)[AlPO4F], (H3N(CH2)4NH3)[Al3(PO4)3F2] and (H3N(CH2)3NH3)[Al3(PO4)3F2] (hereafter noted respectively CJ2, ULM-3 and ULM-4 [ULM stands for University Le Mans]). Their molecular formulas show that all the atoms present in these structures are NMR nuclei. With our apparatus, NMR experiments allowed to follow the evolution of the signal of each atom during the reaction and therefore provided independent information of each particle during the reaction. The 3D structure of AlPO4 CJ246,47 can be described (Figure 3a) from tetramers containing two PO4 groups corner-linked to two Al polyhedra in six- and five-fold coordination respectively. One fluorine atom is shared between the two Al polyhedra ; the second, on the Al octahedron, is terminal. Ammonium ions are inserted in the tunnels. Our NMR study in the range 30-150°C used all the NMR nuclei present in the structure (Figure 3b). The ex situ study concerned the mother liquor above the solid, whereas in situ was dedicated to the solid-liquid mixture. The evolution with time of the signals showed several steps in the formation of the SBU. At 30°C, the Al monophosphate is the only species present (Figure 3c). Al3+ ions are octahedrally surrounded by one oxygen, two OH groups, one H2O molecule and two F- in cis position. When temperature increases, a change in the Al coordination first occurs with a loss of one F-. During the rest of the reaction, five-fold coordinated Al is the reactive species. The new signals of weak intensity which appeared in the

27Al

NMR spectra indicated a dimerization of the

monophosphates complexes associated with the loss of two H2O, in a way depicted in Figure 3c. At the end, this oligomeric condensation is followed through an intramolecular nucleophilic addition establishing Al-F-Al bridges by rotation of the Al-F bond of one of the two Al. This results in the SBU observed in the solid.


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Figure 3. (a) Projection of AlPO4 CJ2 with its tetrameric building unit, (b) in-situ liquid-state 31

P, and

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F,

Al NMR spectra of the synthesis precursor of AlPO4 CJ2, and (c) the main

fluoroaluminate species observed in the synthesis solution. The 1:1 fluorinated Al monophosphate complex (i) adopts an octahedral coordination environment at room temperature prior to heating as revealed by multinuclear NMR. Upon heating the Al monophosphate complex transforms into a 5fold coordination (ii) by loosing one coordinating water molecule. Dimerization of the complexes leads to the terameric PNBU (iii) stable enough in synthesis solution. After an intramolecular nucleophilic addition (blue arrow) involving the rotation of an Al-F bond, the final SBU-4 (iv) is obtained in the final solid. For sake of comparison, the insert shows the different linear SBU-4 (v) existing in some solutions leading to titanium phosphates (see text).

Note that, when studying in the same way the formation of porous titanium(IV) fluorophosphates48 (the chemistry of Ti(IV) in the solution was practically unknown), a different SBU-4 (insert of Figure 3), non cyclic, was evidenced from 19F and 31P NMR in the solutions. It led to the structures of both nanoporous π–TiP and a new hexagonal mesoporous titanophosphate. Oxygen atoms link Ti atoms, and both fluorine atoms are terminal. ACS Paragon Plus Environment

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ULM-349,50 and ULM-4,51,52 synthesized using two different templates, exhibit also a 3D structure 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(Figure 4) with Al or Ga cation. This time, they are both built up from the same vertice connection of hexamers Al3P3 (central insert of Figure 4) instead of the previous tetramers Al2P2. The two external aluminum-centered polyhedra exhibit five-fold coordination; it is six-fold for the central one. However, their SBU connection is different as shown on the figure. With the same procedure as above, the ex/in situ study of their formation was performed. It showed for the different steps53 a close similarity with that of CJ2 with, in particular, the change of coordination from six to five in the monocomplex when temperature increases from 30 to 180°C. The only difference concerns the extent of the oligomeric condensation: a trimerization instead of the dimerization with CJ2. Otherwise, the other steps are identical with, successively, the formation of P-O-Al bridges and, at the end, that of Al-F-Al ones for giving the SBU observed in the resultant solid.

Figure 4. Structure of ULM-3 (left) and ULM-4 (right) constructed from the same hexameric Al3P3 unit (middle). A deeper study of the formation as a function of temperature of the gallium-based ULM-3 templated with 1,3-diaminopropane evidences that, between 30 and 90°C, ULM-3(Ga) is not formed when P4O10 is used instead of H3PO4, whereas ULM-3 immediately appears when phosphoric acid is used as a source of P.54 Instead, with P4O10, gallium pyrophosphates are formed. After 7 days at 30°C, the 1D Ga(P2O7)F, N2C3H12·3H2O crystallizes. The Ga-P chains are those encountered in mineral Tancoïte. The heating of the initial mixture at 90°C for 4 days leads through a topotactic reaction to the monohydrate. Above, after a dissolution-recrystallization process, ULM-3 begins to form according to the mechanism described above. However, the most important result was to observe the influence of the nature of the amine on the nature of the SBU and the final structure. Indeed, what is the situation? In the conditions of reaction, ACS Paragon Plus Environment

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the templating amines are protonated, whatever the size of the SBU. This implies that, at the end, the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

latter must be anionic, whereas the initial Al or Ga phosphate-complexes are known to be cationic.55 The mechanism must explain the transition from the cationic monophosphate complexes in the initial solution to the observed anionic SBU in the final solid. It is in agreement with the above results. Keeping in mind that precipitation of a solid can only occur if neutral species are present in the solution implying both the formation of zero-charged ammonium-SBU associations (hereafter called MBU) and the edification of the solid framework from the infinite condensation of these neutral MBU. This is only possible when electronegativities of both the SBU and the protonated template are equal allowing, according to Henry et al.,56 the mutual approach of the entities without electrostatic repulsion. We stated that the observed anionic condensation mechanism is therefore driven by the lowering of the charge density of the condensed species. For the anionic species, this density will be lowered for SBU with large size, or for the same size of SBU, when the coordination of aluminum or gallium will decrease, and this will be driven by the characteristics of the protonated amine, which is, with its shape and pKa, the only fixed parameter in the reaction. Its charge density is fixed in a wide pH range. The protonation obviously depends on the value of its pKa. At this stage, it is worthy to note the value of the pKa is strongly dependent of the shape of the amine. The spherical amines, which have a pK1 close to 5, are much less basic than the linear amines whose pKa varies in the range 8-10. Their positive charge density is therefore higher than that of the linear amines, the charge density of which decreases when the length of the chain increases. Spherical NH4+ plays a special role here since, despite a high pKa, its small size leads to a high charge density.

The charge density of the amine controls the importance of the oligomeric

condensation and therefore the size of the SBU, up to the equalization of the electronegativities of the groups. The higher the charge density of the amine the lower is the nuclearity of the SBU. This was verified experimentally on many porous solids in this system.57,58 The oligomeric condensation implies olation and oxolation reactions, which transform the cationic monophosphate complexes into anionic SBUs. The equalization of the electronegativities stops the condensation. Due also to the presence of fluorides, the result is the formation of a tight ammonium-


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SBU association which can be compared to a neutral cation-anion pair with zero-charge 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

characteristics allowing the infinite condensation. The topology and the steric occupancy of this ion pair will determine the nature and the characteristics of the resulting structural forms (size and shape of the cavities, connection mode of the SBU...).57,59 In this way, the notion of 'template' must be revisited. The amine is the driving force in the synthesis: (i) the introduction of a given amine in the reaction medium, owing to its pKa, its form and its length, experimentally imposes a known charge density in a system in which all the other species are variable, (ii) it will determine the evolution in size and charge of the SBU until the latter reaches a charge density and an electronegativity equal to that of the considered amine, (iii) it is at the origin of the creation of the neutral ammonium-SBU pair which allow the infinite condensation, (iv) when the pair is formed, the steric effects and the flexibility of the amine fix the adopted structure type, based on a minimum of lattice energy. In particular, if the volume of the amine is too large, it will oblige the structure to become two-dimensional instead of the classical 3D one. It was the case for ULM-8.60 In other cases, 3D frameworks (TREN-GaPO) are formed by using a mixture of amines or amine and alkali cations.61,62 Of course, this mechanism of formation is not a general rule, but just a contribution to academic knowledge. It obviously depends on the characteristics of the metals involved in the synthesis. However, this mechanistic approach was validated, at least for open-framework alumino/gallophosphates with, using these rules, the experimental discovery of many porous solids based on the above hexamer Al3P3 (Figure 5). Beyond the fact that SBUs is now a chemical reality instead of being, as before, just a tool of description, this allows the prediction and computer simulation of possible arrangements based on a given ‘brick’ whose geometry is well known from structural studies. It is what we successfully did.63 However, developments on this topic are out of the scope of this paper.


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Figure 5. Distinct porous alumino/gallo-phosphate phases sharing the same hexameric unit M3P3 (M = Ga; M = Al/Ga for ULM-3 & ULM-4). Polyhedra represent the inorganic network of the solid and stick and ball indicate the occluded extra-framework species including the organic templates (black and blue for C and N), water molecules (red), and anionic species (green for F, grey for cations).

However, this family of inorganic porous solids suffered of a major limitation for possible applications. Due to strong host-guest interactions, the extraction of the template for accessing the pores leads to an amorphization of the solid. We therefore decided to circumvent these limitations and focus our interest on the new family of hybrid porous solids, usually called MOFs (for Metal Organic Frameworks) or PCPs (for Porous Coordination Polymers) with always the aim of understanding their formation.


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5. Formation of porous hybrid materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Since their discovery by R. Robson in 1989,64 these solids, which result from the association of inorganic and organic moieties exclusively linked by strong bonds have known a tremendous development with, up to now, thousands of published new structures (refs10,11,65 and refs therein). As for inorganic porous solids, they are usually obtained using hydro/solvothermal synthesis in a temperature range 100-200°C under autogenous pressure.66 However, despite the extreme richness of this new family, the multitude of published new solids, their multifunctional character, the reasons of their formation remained a mystery. As we did for metal phosphates, we undertook a study to solve these mechanisms in the cases we had encountered. The naive question was always the same: do the inorganic bricks observed in the final solid preexist in the solution before precipitation (Figure 1 (right))? For such a question, some differences had to be noted between the behavior of inorganic and hybrid solids: (i) the solvent acts as a template with, this time, very weak interactions with the skeleton, (ii) the solvent is this time neutral, implying the same neutrality for the framework and the final SBU. Therefore, the strategy approach had to incorporate these facts to the study without forgetting the results obtained with inorganic matrices. Moreover, the different SBU observed in the numerous MOFs also previously existed as clusters in the literature of coordination compounds, synthesized in various conditions.67,68 This gives first an idea of the enormous possibilities for discovering new MOFs with these clusters, and incidentally asks for the question of the formation of coordination compounds and, upstream, the nature and structures of the precursors used for these syntheses. Our study could, at least partially, shed some light on these questions and increase the academic knowledge of the chemistry in the solutions. Two possibilities therefore appear: (i) either the inorganic part found of the final MOF has a structure reminiscent of that in precursor or (ii) no obvious structural relation exists between the precursor and the MOF. In the first case, a subsidiary question arises: is the inorganic brick in the precursor stable enough in the solution for immediately leading without change to its incorporation ACS Paragon Plus Environment

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in the framework of the MOF or does it disappear through a dissolution process and reconstructs at 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the contact with the ligand? In the second case, the different steps of the formation of the MOF must as before be elucidated from the complex ions present in the solution, knowing that the ligand does not exhibit any transformation during the reaction. 5.1. The case of iron and chromium carboxylates MIL-88 and MIL-89. The first case is the simplest. It was illustrated from isostructural MIL-88 and MIL-89 (refs44,69-71 and refs. therein) with flexible structures. They are synthesized using metal(III) acetate as precursor (M = Cr, Fe), in which trimeric clusters of metal octahedra with a central µ3-O oxygen appear. They exist also in the resulting MOFs, suggesting an invariance of the SBU in the conditions of reaction. In order to, we performed a time-resolved in situ diffraction study of the hydrothermal crystallization of Fe(III) fumarate MIL-89, followed by an EXAFS characterization at each step (Figure 6).


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Figure 6. (a) The trimeric cluster of Fe(III) acetate (oxygen : red ; H2O : blue) ; (b) a partial perspective view of the 3D structure of fumarate MIL-89; (c) Powder X-Ray diffraction patterns of the solids isolated from the solvothermal reactions leading to MIL-89 (asterisks indicate Bragg peaks due to unreacted trans-trans muconic acid;(d) evolution of the EXAFS signals (left) and their deconvolution (right).

The synthesis of MIL-89 is achieved after 60 hours. The reaction starts during the first hour by the formation a non-identified amorphous (or nano-crystalline) phase. The Bragg peaks of MIL-89 begin to appear only after 2 hours (Figure 6c). Their intensity continuously increases up to the end of the reaction, indicating first that MIL-89 is the only crystallized phase appearing during the reaction. ACS Paragon Plus Environment

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Moreover, the deconvolution of the EXAFS signals (Figure 6d right) clearly shows that the radial 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

distribution functions remain exactly the same, whatever the progression of the reaction. This clearly evidences, at least in this case, the invariance of the inorganic cluster all along the hydrothermal reaction. Figure 7 illustrates the different steps of the synthesis.

Figure 7. Illustration of the different steps of the formation of MIL-89.

5.2. The case of aluminum trimesates MIL-96, MIL-100 and MIL-110. The second case – which is the most general – is much more difficult to decipher. A good illustration required to select a system within which several compounds exist by varying the chemical conditions of the synthesis. The Al trimesates (1,3,5 – benzene tricarboxylate) are good candidates for such a purpose. Indeed, three phases, with completely different 3D crystal structures, appear in this system (Figure 8) by varying pH and time. The precursors are Al(NO3)3, the easily hydrolysable 1,2,3–methyl benzene tricarboxylate, in water medium, acidified with HNO3. The complex structure of MIL-9672 combines linked corrugated chains forming hexagonal 18-membered ring tunnels, at the center of which appear the trimeric cluster already encountered in MIL-89. This trimer represents the primary building unit in the mesoporous MIL-100,73 whereas MIL-11074 is built up from the association of Al8 octamers.


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Figure 8. Domains of existence as a function of synthesis pH and time of the three aluminum trimesates MIL-96, MIL-100 and MIL-110 with their structural schemes.

At short times (< 5 hours), increasing pH successively leads to MIL-100 in a very narrow range of pH (0.5 < pH < 0.75),73 MIL-96 (0.75 < pH < 3.25)72 and MIL-110 (pH > 3.25).74 Above 60 hours of reaction, the repartition has completely changed: MIL-100 has disappeared and MIL-110, which was formed in 4h at pH 3.5, exists now only at very low pH (< 0.5), MIL-96 domain being almost unchanged. These surprising chemical results deserved to be explained by the combination of X-Ray powder diffraction and our in situ NMR method.75 The three in situ 27Al NMR studies showed almost the same features meaning that identical species were present in their respective synthesis solutions, but their amount and timing of appearance vary depending on the intermediate or final solid product (Figure 9).


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Figure 9. Representative 27Al NMR spectra during the synthesis of MIL-96, MIL-100 and MIL-110 at 180 °C with, below, the aluminum fractions detected by NMR in the solution during the synthesis as three main species: the monomers Al/1:1 Al-(btc) (black), the dimers 2:1 Al2-(btc) (red), and the dimers 2:2 Al2-(btc)2 (blue) complexes.

Four signals can be distinguished at 0, ~ 1, ~ 4, and ~ 7 ppm. They correspond to four distinct Al based species in octahedral coordination. Their identification is based on comparison between NMR observation in solution and the nature of the solid product along the synthesis course of each phase. The signal at 0 ppm is observed in all the studied solutions. It is assigned to uncomplexed cation Al(H2O)63+. The resonance at ~ 1 ppm appear during the increase of the temperature. Its presence is correlated with the presence of benzene-1,3,5-tricarboxylate (btc) in solution, and therefore assigned to the primary complex Al(H2O)5(H2btc)2+. This labile complex undergoes fast chemical exchange with Al(H2O)63+. The signal at ~ 4 ppm mainly concerns the MIL-110 formation; the other (~ 7 ppm) is more correlated with the formation of MIL-96 and, to a lesser extend, of MIL-100. These 4 and 7


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ppm signals would be structurally related to some building units present respectively with MIL-110 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and, on the other hand with MIL-96 and MIL-100. Coming back to their structures, MIL-110 is based on original octamers, whereas similar structural features exist between MIL-96 and MIL-100, for instance, the trimer Al3(µ3-O)(H2O)2(OH)(btc)2. Furthermore, as the 7 ppm contribution appears always after the 4 ppm signal, the two species should be structurally related. Therefore, the substructure Al2(µ2-O)(H2O)2(btc)22- (corner-sharing bi-octahedral motif) would be more likely related to the 7 ppm signal, knowing that MIL-110 presents another kind of dimer Al2(µ2O)2(H2O)2(btc)- (edge-sharing bi-octahedral motif), that would be related to the 4 ppm signal. On this basis, the resonances at 4 and 7 ppm are assigned to the dimers complexes Al2(µ2OH)2(H2O)6(H2btc)3+ and Al2(µ2-OH)(H2O)6(H2btc)23+ respectively. For an easier reading, the three Al species will hereafter be noted 1:1, 2:1 and 2:2 indicating each time the Al:btc ratio in the complexes. For identical conditions of similar systems but without btc ligand, the study does not evidence these 4 and 7 pm contributions, and therefore confirms our assignments of the NMR signals (~ 1, 4 and 7 ppm). 5.2.1. The initial chemical reactions and equilibria between aluminum and trimesates in solution. Based on both the latter and their various sequential evolutions with time (Figure 10), a general scheme can be proposed for the first chemical reactions and equilibria taking place in the reactive solutions.


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Figure 10. Correlation between evolution with synthesis time of nature of the solid phases and the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

evolution of the aluminate species in synthesis solution as revealed respectively by ex situ XRD and in situ liquid state

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Al NMR at 180 °C in two separate syntheses: synthesis I & II optimized

conditions for MIL-110 and MIL-96 respectively. The darker each color, the stronger the intensity of the diffraction peak or the NMR signal.

Whatever the phase and reaction time, the Al monomers and 1:1 primary complex, are present in all solutions during all the syntheses. They are the main precursor species in the early stages of the synthesis. The 2:1 complex (4 ppm) appears always first within the first hour of heating. The 2:2 one (7 ppm) dimer with two btc ligands, Al2-(btc)2, appeared later after 2 hours, but only in the case of syntheses of MIL-96 and MIL-100. Therefore, one can conclude that the first dimer is a precursor for the second. Since the btc ligand is produced in situ through the slow hydrolysis of the trimesate ester at the beginning of the synthesis, 2:1 is formed first. Only when btc is produced enough, through the slow hydrolysis of the ester, 2:2 is obtained. Due to the edge connection of the Al octahedra, it is more stable than 2:1, the latter being never observed at 25°C. The structures of these complexes and their relationships are illustrated in Figure 11.

Figure 11. Structures of the of the primary complex Al(H2O)63+ and of the complexes 1:1 (Al(H2O)5(H2btc)2+), 2:1 (Al2-(btc)1) and 2:2 (Al2-(btc)2). See text.


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5.3.2. Mechanisms of the early stage of formation of MIL-96, MIL-100, and MIL-110. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Assignments of 27Al signals are based on comparison between ex situ and in situ characterization of respectively solid and liquid parts along the synthesis. According to our SBU concept,58 the species present in solution at the moment of crystallization are directly related with the building units of the crystal. In this way, the dimeric complexes 2:1 and 2:2 cannot be regarded as SBU species. Their strict associations cannot lead alone the structure of each phase. However, they can be considered as prenucleation building unites (PNBU) because they act as precursor species for the final SBUs, which further react with Al monomers for giving the SBU. Figure 12 shows how each possible SBU of each phase can be obtained from the reaction of the observed dimeric btc complexes with Al(H2O)63+ in the solution.

Figure 12. For each solid, illustration of the step by step formation of the final SBUs through the reactions between 2:1 and 2:2 PNBUs with supplementary Al(H2O)63+, See text. ACS Paragon Plus Environment

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In each case only monomer cation Al3+ is needed to join the dimer btc complexes to form the SBU (or crystal integrate unit). For MIL-110, the octameric unit Al8(OH)15(H2O)3(btc)3 results from the combination of three 2:1 complexes with two Al3+ cations. The trimeric SBUs of MIL-100 can be decomposed into one Al3+ cation and one 2:2, whereas MIL-96, based on the assembly of the Al12O(OH)18(H2O)3(btc)6 unit, originates from the reaction in the solution of six Al3+ cations and three 2:2. This study illustrates the four successive steps: formation of zero charged soluble precursors, nucleation, crystal growth, and dissolution-recrystallization equilibriums, governing the formation of solids from solution.45,76 Hydrothermal conditions favor condensation process by lowering the dielectric constant of the solvent. Species in solution tend to minimize their global electrostatic charge until producing neutral charge complexes suitable for infinite condensation. Nucleation takes place after supersaturation of the zero charged species that condense when reaching their critical concentration. In our in situ NMR experiments, the sudden decrease of the signals intensity of the dimeric Al2 complexes just after reaching its maximum reflect the supersaturation/nucleation phenomena. When reaching the required concentration of PNBUs, nucleation occurs immediately after zero charged complexes formation. This implies that the latter are unstable in an isolated form and hence, unobservable because their lifetimes are very short. However, once inside germs network, their stability increases by minimizing the lattice energy of the crystallites, which, in our experiments, appear as spherical shape particles usually with diameters < 1 µm. The crystal growth is then a slow and continuous process. Aging of the dispersed particles in their mother solution results in a thermodynamic equilibrium during which dissolution-recrystallization occurs. This process allows not only size growth of the crystal but also appearance of the final crystal shape with well-defined faces and edges. It was typical for MIL-100. After 1 h of reaction, the solid products appear as aggregated spherical nanoparticles, corresponding to the primary germs. After 8h, dissolution-recrystallization phenomena lead to purely octahedral crystals of MIL-100.


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6. Pragmatism and consequences

Whatever the class of porous solids submitted to our investigation, the above results seem at first glance rather frustrating. Beyond the fact that we have evidenced the features governing the formation of porous solids: (i) the dissolution of the precursor giving rise to ionic species; (ii) a complex behavior within the solution resulting, in the majority of the cases, from several oligomeric condensations of the inorganic part for reaching by equalization of the electronegativities the formation of neutral ion pairs which lead to nucleation and growth of the solid phase, and that we have proved the existence of the reactive species, in relation with SBU. Each case is a particular case, depending on the nature of the metal and of the pH of the reaction. Each metallic cation has its own chemistry, which governs the oligomeric condensations occurring before the precipitation of each MOF. At least, our work is a pioneer contribution to the knowledge of the formation of porous solids. Frustration, but also pragmatism. Indeed, useful conclusions can however be extracted for this investigation. The formation of crystal proceeds following two distinct steps: nucleation or “density fluctuation” resulting from aggregation of neutral pairs of ions, or molecules, and crystal growth through bi-dimensional addition of basic units to already existing crystallites. The SBU concept is a simple way of description of solids by comparing SBU of the crystal to the PNBU present in solution in desolvated form and considering template effect. In recent papers, Blatov et al.77,78 introduced the Natural Building Units (NBU), to identify building schemes for microporous solid construction using computational algorithm. The approach also allows enumeration of all possible common building units of existing zeolitic structures. The SBU/NBU should exist only in the crystalline framework, and its relationship with the species present in solution imply desolvatation step first, and then stabilization into a specific conformation imposed by the template. Because these two consecutive processes are necessarily energy demanding, the PNBU in solution should be structurally different from the NBU. The stability in solution is driven by the formation enthalpy and ACS Paragon Plus Environment

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solvatation energy. The removal of solvent should cost as high as the necessary lattice energy and 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

should be accompanied by conformational changes with or without new bonds formation. The gain in bulk lattice energy should be the same for a given topology with different chemical elements but the free energy should be different. This situation is encountered in the case of MIL-100 where the enthalpy energy for the basic trimeric unit formation is dominant for the chromium and iron isotypes, whereas the entropic contribution dominates for aluminum-based compound. For this reason, the trimeric units exist in solution for the two formers, but only in the crystal for the latter. The chemical knowledge of the conditions of synthesis and the structure of a given new MOF implies that the SBUs chosen for building up this structure is derived from PNBUs existing in the solution before the precipitation, in the conditions applied to the synthesis, including pH which is a major parameter. Fixing the same conditions for the search of isostructural phases implies that the resulting structures will exhibit the same inorganic bricks, except if the ligand is too different/large for corresponding to the same lattice energy minima. This is very important for industry researchers for an optimum time of research. A recent example illustrating this point is provided by BASF,79 which from our results on Al MOFs, produces now a porous Al fumarate efficient for storing methane in it, and propose cars alimented with natural gas. The last pragmatic incidence of our results corresponds to the prediction of new porous structures. As the SBU is known chemically and structurally as well, computer simulations dedicated to the different possibilities of connection between these SBUs. They were performed for inorganic porous solids with our AASBU program63,80 or others such as TOPOS developed by Blatov et al.77,78 We made a powerful adaptation for MOF solids.81 In particular, it was able to predict as soon as 2004 the structures of MIL-100 and MIL-101. In the absence - at that time - of single crystals, their crystallized mesoporous organization would not have been reached from powder diffraction without 3

this prediction using the program. Indeed, their huge cell volumes (ca. 400,000 and 700,000 Å respectively) are far beyond the possibilities of resolution of synchrotron radiation experiments.


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Nanoporous Solids: How Do They Form? An In Situ Approach

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