Recent advances in application of 27Al NMR spectroscopy to materials science

Accepted Manuscript Recent advances in application of 27Al NMR spectroscopy to materials science Mohamed Haouas, Francis Taulelle, Charlotte Martineau PII: DOI: Reference:

S0079-6565(16)00004-2 http://dx.doi.org/10.1016/j.pnmrs.2016.01.003 JPNMRS 1415

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Progress in Nuclear Magnetic Resonance Spectroscopy

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Please cite this article as: M. Haouas, F. Taulelle, C. Martineau, Recent advances in application of 27Al NMR spectroscopy to materials science, Progress in Nuclear Magnetic Resonance Spectroscopy (2016), doi: http:// dx.doi.org/10.1016/j.pnmrs.2016.01.003

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Recent advances in application of 27Al NMR spectroscopy to materials science Mohamed Haouas,* Francis Taulelle, Charlotte Martineau Institut Lavoisier de Versailles (UMR CNRS 8180), Tectospin Group, Université de Versailles Saint Quentin en Yvelines, 78035 Versailles, France AUTHOR INFORMATION Corresponding Author *(M.H.) E-mail: [email protected].

ABSTRACT Valuable information about the local environment of the aluminum nucleus can be obtained through 27Al Nuclear Magnetic Resonance (NMR) parameters like the isotropic chemical shift, scalar and quadrupolar coupling constants, and relaxation rate. With nearly 250 scientific articles per year dealing with 27Al NMR spectroscopy, this analytical tool has become popular because of the recent progress has made the acquisition and interpretation of the NMR data much easier. The application of 27Al NMR techniques to various classes of compounds, either in solution or solid-state, has been shown to be extremely informative concerning local structure and chemistry of aluminum in its various environments. The development of experimental methodologies combined with theoretical approaches and modeling has contributed to major advances in spectroscopic characterization especially in materials sciences where long-range periodicity and classical local NMR probes are lacking. In this review we will present an overview of results obtained by 27Al NMR as well as the most relevant methodological developments over the last 25 years, concerning particularly on progress in the application of liquid- and solid-state 27Al NMR to the study of aluminum-based materials such as aluminum polyoxoanions, zeolites, aluminophosphates, and metal-organic-frameworks.

__________________________________________________________________________________________ Keywords : 27 Al nucleus Chemical shifts Scalar coupling Quadrupolar relaxation Solution chemistry Materials

1.

Introduction

2.

NMR properties and parameters of 27Al nucleus 2.1.

Chemical shifts 2.1.1.

Compounds with oxygen-donor ligands

2.1.2. Compounds with halide ligands 2.1.3. Compounds with sulfur-donor ligands 2.1.4. Compounds with nitrogen-donor ligands 2.1.5. Compounds with phosphorus-donor ligands 2.1.6. Aluminum hydrides and alkylaluminums 2.2.

Scalar couplings

2.3.

Nuclear spin relaxation and quadrupole parameters

2.4.

Quantification and “NMR invisible” aluminum

3.

Experimental and theoretical methods and techniques 3.1.

Experiments under MAS

3.2.

Distance measurements and correlation experiments 3.2.1. Measuring J couplings in solids 3.2.2.

Correlation experiments

3.2.3.

Distance measurements

3.3.

High magnetic field

3.4.

DNP measurements

3.5.

Modeling and theoretical calculations

3.5.1.

Chemical shielding calculation

3.5.2.

J-coupling constant calculation

3.5.3.

EFG and quadrupolar parameters calculations

4.

Examples of applications 4.1.

Aluminum(III) cation in aqueous solution 4.1.1.

Relationship between

27

Al chemical shifts and Al–O distance in AlO4

environments 4.1.2.

Conversion of -Al13 into Al30

4.2.

Aluminum in polyoxometallate compounds

4.3.

Aluminosilicates and zeolites

4.4.

4.3.1.

Speciation in zeolite synthesis precursors

4.3.2.

27

Al solid-state NMR of zeolites

Aluminophosphate zeotypes 4.4.1.

Hydrothermal NMR to study formation mechanisms of oxyfluorinated

aluminophosphates 4.4.2. 4.5.

5.

Solid-state NMR of AlPOs

Metal-Organic Frameworks (MOFs) 4.5.1.

Formation mechanisms of MOF type aluminum trimesates

4.5.2.

Solid-state 27Al NMR of MOFs

Concluding remarks Acknowledgements References

1. Introduction Aluminum is the most abundant metal in the earth’s crust, and it is therefore found in a large number of chemical compounds. The structural characterization of aluminum-containing compounds by

27

Al nuclear

magnetic resonance (NMR) spectroscopy, either in the liquid-state or in the solid-state, has become popular because of the recent progresses that nowadays make the acquisition and interpretation of the corresponding NMR spectra much easier. NMR is a short-range technique that can provide, through the determination of NMR parameters like the isotropic chemical shift, scalar or dipolar coupling constants, quadrupolar parameters and relaxation behavior, valuable information about the local environment of the aluminum nucleus. These pieces of information can be combined with insights coming from other analytical or computational techniques. In particular, in the solid-state, NMR has proven complementary to X-ray or electron diffraction and infrared spectroscopy, with the development of the concept of “NMR Crystallography” [1, 2]. Since the general review of Akitt on the NMR studies of aluminum compounds mostly in solution [3], several reviews have been published on the application of

27

Al NMR spectroscopy to various classes of

compounds such as biologically relevant aluminum complexes [4, 5], organoaluminums [6, 7], soils [8, 9], mineral solids [10, 11], and solid catalysts [12, 13]. If the fundamental principles of liquid-state NMR measurements have not changed much since 1989, the solid-state NMR of quadrupolar nuclei (i.e., nuclei with nuclear spin >½) has undergone major improvements in which solid-state 27Al NMR spectroscopy is used. In particular, technical advances (higher magnetic fields, faster magic-angle spinning (MAS) friquencies) and methodological developments (multiple-quantum magic-angle spinning (MQMAS), multidimensional NMR, etc.) have rendered solid-state NMR spectra easier to read, understand and interpret. Like in 27Al NMR spectra of solution-sate samples, it is nowadays quite easy to determine

27

Al chemical shifts and quadrupolar

parameters (quadrupolar coupling constant Cq and asymmetry parameter η) from a single-pulse or a MQMAS NMR spectrum. Other information about connectivity between a 27Al nucleus and other nuclei (27Al, 29Si, 1H) can also be readily available, and provide relative distance information on a local scale. These advancements have opened a new way of using

27

Al NMR spectroscopy for the structural characterization of numerous

crystalline or amorphous Al-containing solids, like organoaluminums, zeolites, molecular sieves, metal-organic frameworks, etc. [14, 15]. Another major advance that is largely responsible for the popularity of 27Al solid-state NMR spectroscopy is the development of density functional theory (DFT) based computer programs that calculate, from a finite or

infinite structure, NMR parameters (full tensors of the chemical shifts and quadrupolar interactions, along with their relative orientation in the compound). This provides a tool to assign resonances and to better understand the 27Al NMR spectra of complex or disordered solids [16, 17]. The present review deals with the use of 27Al NMR spectroscopy, in both solution and solid state, of the last 25 years (1990–2015). After a presentation on the 27Al nucleus and the information that can be extracted from 27

Al NMR spectra, applications of the technique to the characterization of a wide variety of compounds are

presented. Since studies involving 27Al NMR spectroscopy are too numerous (more than 6000 papers since the last 25 years) to be all included here, we have chosen to restrict the review to some representative examples of the most relevant aluminum chemistry in the field of materials science.

2. NMR properties and parameters of 27Al nucleus

The element aluminum (Al) is magnetically active and presents a unique isotope, that is, aluminum-27, with a good receptivity (D = 1170 relative to 13C). As the 27Al nuclear spin I = 5/2 is >1/2 (quadrupole moment Q = 0.14 × 10−28 m2), this nucleus interacts not only with the external magnetic field B0, but also with the electric field gradient (EFG) generated by its surrounding environment. This leads to typically broadened and often overlapping NMR resonances, composed of a central transition (CT) +1/2 ↔ -1/2 only affected by the secondorder quadrupolar interaction, flanked by satellite transitions (STs) also affected by the first-order quadrupolar interaction. As an illustrative example of such effects, one can compare 27Al NMR to 29Si NMR (I = 1/2, D = 2.09 relative to

13

C) of aluminosilicate solutions (Fig. 1), which are characterized by coexisting ensemble of

aluminosilicate oligomers of various degrees of local connectivity around the tetrahedral Si and Al atoms. The resonances of the Qn silicates (SiO4−n(OSi)n) and qn aluminosilicates (AlO4−n(OSi)n) can be identified in these respective NMR spectra on the basis of connectivity effects on the chemical shift showing some analogies between 27Al and 29Si NMR. In the 29Si NMR spectrum, the resonances are narrow (linewidths <1 Hz) allowing straightforward identification and distinction of the individual sites/species whereas in the 27Al NMR spectrum, only one representative resonance for a given environment type can be observed. If the 27Al NMR spectrum suffers from lower resolution, it however benefits from higher sensitivity (better signal-to-noise ratio).

Figure 1

Valuable information on structural, chemical, and dynamic properties of aluminum compounds may be derived from spectral parameters of 27Al nuclei such as chemical shifts, spin–spin couplings, linewidths, and relaxation times. In addition to chemical shift and scalar interactions, quadrupolar interactions have to be taken into account. The six (2I + 1) energy levels created by the Zeeman interaction are indeed affected by the first-order interaction of the nucleus quadrupole moment (eQ) with the EFG (eq), which results in five distinct transition frequencies. The resulting quadrupole coupling constant (e2qQ/h) is usually much larger than the dipolar and scalar interactions. The line broadening due to second order quadrupolar interaction is proportional to (e2qQ/h)/L, where L is the Larmor frequency of the 27Al nucleus. The two main dominating and accessible parameters are the isotropic chemical shift and the quadrupolar line broadening. These parameters can be extracted easily from a one-dimensional (1D)

27

Al NMR spectrum in case of liquids or

solutions, while in case of solids it is more challenging and a two-dimensional (2D) spectrum is often required for this purpose. If the principal relaxation mechanism mainly depends on the EFG around the nucleus, molecular tumbling can also cause a fluctuating EFG leading to efficient quadrupolar relaxation even for highly symmetrical environments.

2.1. Chemical Shifts The 27Al chemical shift range, in both liquid and solid state, is relatively wide, covering nearly 400 ppm from ca −100 to 300 ppm relative to the reference, Al(H2O)63+ at 0 ppm. One of the main pieces of structural information that can be deduced directly from the chemical shift is the coordination number of the aluminum cation as shown in Fig. 2. for different classes of compound either in solution or in the solid state. The general trend shows that aluminum atoms occurring in an octahedral environment are usually characterized by higher field resonances than those of the tetrahedral aluminum, while five-coordinate complexes exhibit intermediate shifts. Furthermore,

27

Al signals from the few known three-coordinate alkyl aluminum

compounds appear at the lowest fields range, whereas those exceeding the coordination number of six (with cyclopentadienyl ligands) resonate at the highest fields supporting the general trend of chemical shift dependency on the coordination number. It clearly appears that the 27Al chemical shift is principally influenced by the electronegativity effect of the ligand and reflects the change of the electronic density in the p bonding orbitals. The paramagnetic contribution of the chemical shift depends mainly on the p electron unbalance and the orbital radius term p. The screening strength naturally increases with the number of bounding ligands although bond lengths shorten. The more the aluminum cation is coordinated with donor ligands, the stronger the shielding around the aluminum center.

Figure 2

The most frequently encountered ligands in aluminum chemistry include hydrides, halides, as well as O-, S-, N-, P-, C-, and Si-donors. Typical 27Al chemical shift ranges observed in different types of compounds of these families, either in solution or solids, are summarized in Table 1. The oxygen-donor ligands cover a wide variety of chemical classes either minerals (phosphates, perchlorates, sulfates, etc.) or organics (phosphonates, alkoxides, carboxylates, etc.). Fluorides, chlorides, bromides, and iodides constitute the aluminum halide family that can exist in both aqueous and organic solutions, but also in the solid mineral form in the case of fluorides. Sulfides can exist in solution as adduct complexes with sulfur-containing solvents. The best known N-donors in solution chemistry of aluminum are mainly cyanide, pyridine, and amine based complexing ligands. Phosphines are the best known P-donor ligands. Aluminum based ceramics are also known to occur in solid state as sulfides, nitrides, or phosphides. Hydrides, alkyls and silyls account for major compounds in the organoaluminum family.

Table 1

2.1.1. Compounds with oxygen-donor ligands The chemistry of aluminum with oxygen atoms in its first coordination sphere is very rich and diverse, with aluminum atoms occurring in a variety of classes of compounds including (hydr)oxides/aluminates, carboxylates/alkoxides, and phosphates/silicates/borates among others. These products have been extensively studied either in molecular or finite polymeric soluble complexes form or as infinite network in solids. The speciation of Al(III) in aqueous solutions over a wide range of pH has been subject to numerous studies by 27Al NMR spectroscopy, which has been proved to be a technique of choice [8-10, 13]. Although a wide number of polymeric aluminum cations up to a nuclearity of 54 was proposed to coexist in hydrolyzed aluminum solutions [8], only a few aluminum species have been firmly identified by 27Al NMR. Indeed, 27Al NMR can monitor only highly symmetric or moderately distorted Al complexes/sites (i.e., sites for which the

effects of the quadrupolar interactions are small enough) whose chemical shifts are as follows: 0–4 ppm for monomers Al(OH)n(H2O)6−n(3−n)+ (n = 0–2) [49], 63 and 76 ppm for the central tetrahedral AlO4, respectively, in the - and -isomer tridecamers AlO4Al12(OH)24(H2O)127+, or Al13 [50-53], 70 ppm for the central tetrahedral AlO4 in Al2O8Al28(OH)56(H2O)2618+, or Al30 [50, 52], and 80 ppm for the tetrahedral anion Al(OH)4– [49]. Nevertheless, there are a couple of more or less resolved signals within the octahedral range 2–8 ppm and the tetrahedral range 40–81 ppm, that they are still unidentified. They could correspond to the dimer Al2(OH)2(H2O)84+ and trimer Al3(OH)4(H2O)105+ [8], for the former and to some Al13 isomers and/or to polymers with higher nuclearities for the second [54, 55]. Similarly, the

27

Al solid state NMR spectra of different polymorphic phases of alumina and their

intermediate states as a function of thermal treatments exhibits three resonance ranges at 0–14, 27–37, and 45–70 ppm assigned to hexa-, penta-, and tetra-coordinated Al sites respectively [11, 56-58]. In oxy(hydroxyl)fluorides, the

27

Al MAS NMR spectra were used to probe and quantify the various AlOx [59],

AlFO4− [60], AlF6−x(OH)x [61, 62] or AlF6−x(H2O)x species. In this latter case, the presence of vacancies could also be detected [63]. For compounds with phosphorus atoms in the second coordination sphere of the aluminum (Al–OP) such as phosphates and phosphonates, the

27

Al shift ranges are typically 20–25 ppm below the shift ranges of

hydroxyaluminates for a given coordination number of the aluminum (Table 1). This shift is associated to the phosphorus decreasing the p-character of the Al–O bond. The interactions of Al(III) in solution with (poly)phosphates, phosphonates, and phosphorylated amino acids are an important issue in biochemistry. Species formed in such biological systems (acidic to neutral pH, low concentrations, etc.) usually adopt an octahedral environment with typical

27

Al shifts in the range −1 to −9 ppm [64-66]. In aluminophosphate

materials, either crystalline or amorphous, 27Al MAS exhibits resonances ranging from −25 to −10 ppm, 6 to 15 ppm, and 22 to 45 ppm for hexa-, penta-, and tetra-coordinated species, respectively [11, 67, 68]. Aluminosilicate polymers in solution are mainly found under alkaline conditions where the solubility is maximal. 27Al NMR spectra of these solutions generally show a maximum of five broad bands within the range 80 to 53 ppm [69-77], which have been assigned to environments denoted as qn, where q refers to four oxygen atoms bonded to tetrahedral Al and the superscript n (n = 0–4) indicates the number of bridges to neighboring Si [78-82]. This notation parallels that used for pure silicate species Qn where the superscript indicates the number of siloxane bridges to a given silicon center. The implicit assumption is that replacement of OH or O− groups by OSi around a central Al results in a substantial change of the 27Al chemical shift to low

frequency, just as replacement of OH or O− groups by OSi around a central Si causes a well known decrease in the 29Si chemical shift. However, the former effect appears to be significantly less (ca 5 ppm) than the latter (ca 10 ppm). Zeolites, aluminosilicate minerals and clays, glass, melt, and fiber aluminosilicates have been extensively studied by solid-state 27Al MAS NMR and resonances in the range 0–8, 30–39, and 42–75 ppm are usually observed for octahedral Al(OSi)6, pentacoordinated Al(OSi)5, and tetrahedral Al(OSi)4 sites respectively [11, 83, 84]. 27

Al NMR spectroscopy has become an established technique to examine the interaction of aluminum with

biological systems such as proteins. Carboxylate functions were found among the binding sites and complexes formed are exclusively octahedral with resonances in the range −7 to 2 ppm [5]. By using smaller model complexing multidentate and chelating ligands like aspartic acid, monohydroxamic acids, salicylic acid, and catechol,

27

Al resonances appeared noticeably at much lower field, that is, in the range 10–37 ppm for

octahedral complexes and 50–60 ppm at high pH (>9.5) where tetrahedral complexes are formed [65]. Pentacoordinated complexes with these groups of ligands are rather rare in aqueous solutions. In organic solvent, pentacoordinate aluminum are however more frequent and the dichloroaluminum acetylacetonate in CH2Cl2 solution showed a shift at 37 ppm [28]. This falls in typical chemical shift range (32–37 ppm) observed in solid state for pentacoordinated Al in carboxylate-based metal organic framework (MOF) materials [27]. 2.1.2. Compounds with halide ligands The affinity of aluminum toward halides is among the highest to form aluminum complexes and aluminum fluorides, chlorides, bromides, and iodides are known to exist in different forms including soluble molecular compounds, melts, and minerals. Fluoride and chloride compounds are the most common products encountered and have been extensively investigated by liquid and solid-state NMR spectroscopy. In aqueous solutions under a variety of concentration and pH conditions, only six-coordinate octahedral fluoroaluminate complexes are present characterized by a 27Al resonance at ca −1 ppm [85, 86]. The direct structural proof of the tetrahedral [AlF4]−, occurring in organic solutions only, was given by Herron and coworkers [87]. Tetracoordination in organoaluminum fluorinates is well established and documented [6]. The 27

Al NMR spectra of the tetrahedral alkylfluoroalanate anions [RnAlF4−n]− showed a progressive downfield shift

as more alkyl groups are added (increasing n), from 49 ppm for the tetrafluoride (n = 0) up to 152 ppm for the monosubstituted fluorides (n = 3) [87-91]. This contrasts with the situation for aquafluoro complexes where little effect is observed on water/fluoride substitution [92]. The 27Al signals of fluoroaluminate species [AlF4]−, [AlF5]2−, and [AlF6]3− in NaF/AlF3 melts [93, 94] move upfield with increasing coordination number following the

general trend observed in fluorine containing crystalline solids and amorphous glasses, where chemical shift ranges are typically 38 to 50, 21 to 22, and −14 to 4 respectively [11, 95]. In the solid-state, similar chemical shift ranges are observed for purely inorganic fluorides [96, 97]. For γ-alumina with fluorine grafted on the surface, new resonances at −17 ppm were observed, attributed to the formation of AlF3.3H2O [98-101]. As in case of O-donor complexes, halide aluminate compounds, with the exception of iodides, provide distinct chemical shift ranges as a function of coordination number with increasing shift toward high field when increasing coordination number. The tetrahedrally coordinated iodoaluminate species [AlI4]− presents an exceptional high field resonance appearing at −27 ppm, and iodine derivatives exhibited particularly shielded resonances [3]. Bromides and chlorides seem to provide similar effect on

27

Al shifts, and compared to

(hydr)oxoaluminates Al(OH)n and Al(OAl)n they have tendency to shift resonance ranges toward lower field. Fluorides however present the reverse effect, that is, their shifts appeared more shielded than for their equivalent aluminates with O-donors. This could be explained in terms of electronegativity effect of strong electron donor ligands such as phosphates, perchlorates, and sulfates. 2.1.3. Compounds with sulfur-donor ligands In sulfur containing solvents like sulfur dioxide and thioethers, aluminum complexes can undergo adduct formation with Al–S bonds. Such a bonding induces ca 10–40 ppm low-field shift of 27Al signals when replacing oxygen based analogue ligands. For instance, the 27Al chemical shifts of the adducts Me2S∙AlCl3 and Me2O∙AlCl3 appeared at 104 and 95 ppm, and for those of Et2S∙AlEt3 and Et2O∙AlEt3 at 221 and 178 ppm, respectively [3]. While there are very few reports in recent literature dealing with solution state 27Al NMR studies, some data are available on aluminum sulfides in solids. Aluminum sulfides can exist as crystalline powders, solid solutions, or amorphous glasses. In these materials, the 27Al signal again appears at higher chemical shifts with respect to oxide compounds. Indeed, signals between 15 and 25 ppm have been observed for octahedral [AlS6] sites and between 100 and 125 ppm for tetrahedral [AlS4] sites [34, 35, 102]. Possible fivefold coordination can also be considered in between these two ranges [34], but firm identification of the structure is still lacking. 2.1.4. Compounds with nitrogen-donor ligands

Aluminum interacts readily with amines, pyridines and cyanide derivatives through the lone electron pair of the nitrogen. Aluminum nitride ceramics are also a known class of materials where Al is directly bounded to the nitrogen atom. In spite the fact that the Al3+ ion has no important biological role, its interaction with model chelating agents, including nitrogen-containing multidentate ligands to mimic binding sites in proteins, was the subject of numerous 27Al NMR spectroscopy studies [4]. The chemical shifts of the 27Al signals due to Al3+ bound to protein transferrin were found to be within the range +40 to −46 ppm, which is in accordance with a sixcoordinate (octahedral) Al3+ complex [103, 104]. The metal ion interacts preferentially with the N-site of the protein when carbonate serves as the synergistic anion, but some O-site and carboxylate interactions are also possible. Organo-aluminum compounds with N-donor functionalized ligands can occur with reduced coordination number in organic solutions. For instance, the tertiary amine aluminum hydride adducts of type (R3N)2-AlH3 showed pentacoordinated Al complexes characterized by 27Al NMR signals in the range 103–109 ppm in benzene solution [39, 40]. With trialkyl alanes, the same N-ligand interacts with the Al center with only one bound leading to tetrahedral Al environment characterized with resonances in the range 178–206 ppm [11, 56-58]. In complexes where Al is bonded only to nitrogen atoms, such as azaaluminatranes,

27

Al resonances of

AlN4, AlN5, and AlN6 were reported to appear around 100, 50, and 0 ppm, respectively [41]. This increase of shielding with increasing coordination number is similar to that observed with AlO x and AlSx (x = 4 to 6) environment types. Solid state NMR of aluminum nitride ceramics also showed comparable ranges where resonances at 100–108, 33–52, and 0–6 ppm have been seen for AlNx units for x = 4, 5, and 6 respectively [35, 42]. The AlN hexagonal wurtzite structure showed an isotropic chemical shift at 114 ppm and the low field shift by comparison to the resonance of AlO4 equivalent unit is explained in term of increased p-character of the Al–N bonds relative to the Al–O [11]. Mixed nitride-oxide environment around tetrahedral Al (AlN4−xOx) could be also detected by 27Al giving rise to intermediate chemical shifts between 108 and 75 ppm [11]. 2.1.5. Compounds with phosphorus-donor ligands Like N-donor ligands, aluminum forms adducts with P-donors through the lone pairs on the phosphorus atom. Phosphines are among the best known ligands. Aluminum complexes with bidentate amido phosphine ligands (NP) showed 27Al resonances at 151–159 ppm for a series of tetrahedral dialkyl complexes (NP)AlR 2 and at 94–99 ppm for the analogous dichloride derivatives (NP)AlCl2 [45]. The difference in chemical shifts indicates an increased p-character bond in the dialkyl complexes compared to their dichloride counterparts

(electronegativity: C 2.55, Cl 3.16). Consequently, with a more electrophilic aluminum atom in chloride derivatives, five-coordinate adducts are readily formed in polar solvents such as THF. This is suggested by their 27

Al NMR spectra which contain resonances in the range 69–74 ppm for (NP)AlCl2(THF), while the dialkyl

complexes did not associate coordinating solvents such as THF or diethyl ether and remained in their original tetrahedral form [45]. With tridentate amido diphosphine ligands (NP2), bearing an additional phosphine moiety, the aluminum complexes adopt pentacoordinated geometry with dichloride derivatives and also with dialkyl compounds in most cases [43]. The observed

27

Al NMR signals for the complexes (NP2)AlR2 and

(NP2)AlCl2 fall within the expected shift range from 54 to 70 ppm for five-coordinate complexes for this class of ligands. Rapid fluxional structural changes in coordination environment from five to four and vice versa have also been found in solution by

27

Al NMR especially with bulky alkyl derivatives. In some cases, fourfold

coordinated Al complexes were observed in solid state by X-ray diffraction (XRD) analysis but appeared as fivefold Al complexes in solution [43]. Very few 27Al NMR data can be found in the recent literature about aluminum phosphide materials. To the best of our knowledge, no new publication except some theoretical studies [105] have appeared since the last paper in the field by Oldfield and coworkers in 1988 [106]. The resonance of the tetrahedral Al sites in AlP semiconductor appeared at 142 ppm [106] and falls perfectly into the chemical shift range for tetrahedral aluminum phosphines in solution (Table 1). 2.1.6. Aluminum hydrides and alkylaluminums Aluminum alkyls and hydrides are among the most common organoaluminum compounds and have a rich and diverse solution chemistry. One of the particular structural features in this family of compounds is the enlarged coordination number range beyond the classical four to six, varying from three to up to seven (or even more). Compounds with aluminum atoms bonded to alkyl are often dimeric/polymeric and thus contain bridging groups that are associated with increased valence of the aluminum center. With bulky alkyl groups this polymerization is generally hindered and aluminum stays in its three-coordinated state characterized by particularly deshielded

27

Al resonances (220–276 ppm) [3]. Otherwise, dimeric alkylaluminums provide

27

Al

NMR signal within a wide range of chemical shifts 221–103 ppm, but some exceptions can occur, as in the case of the red [(Me2AlNC5H4(Me))2]2− showing a high-field resonance at 44 ppm [107]. Pentacoordinated centers in alkylaluminums present signals in the usual range 135 to 52 ppm. Octahedral alkylaluminum complexes are rather rare, but in cyclopentadienylaluminum derivatives high coordination numbers can be reached. The 27Al

chemical shifts for the 5-cyclopentadienylaluminum compounds are generally at higher field than what one would expect based on the effective coordination number of the aluminum and are very sensitive to the nature of the other substituents on the aluminum. Seven-coordinate compounds exhibit chemical shifts ranging from −14 to 65 ppm [7]. The aluminocenium cations Cp2Al+ and Cp2*Al+ have the highest field 27Al NMR chemical shifts yet observed for normal-valent organoaluminum compounds [108, 109], which could be at least partly attributable to ring current phenomenon [7]. In the aluminum carbide Al4C3, the aluminum atom is surrounded by four carbon atoms and its 27Al solidstate NMR spectra showed resonances between 111 and 120 ppm [110]. These values for a local AlC4 environment are close to the values observed for AlN4 and AlS4 in solid state, but higher than for AlO4 in oxides and lower than for AlP4 in phosphides (Table 2). The trend of chemical shift for tetrahedral Al as a function of the nature of the first coordination shell atom, that is, (AlP4) > (AlC4) ~ (AlS4) ~ (AlN4) > (AlO4), seems to follow the electronegativity change: the higher the electronegativity, the higher the resonance field. Nevertheless, the deshielding effect of the first contact atom on the aluminum center increases with the electronegativity (from P to O), which also reflects the change in the bond character from covalent to ionic.

Table 2

The experimental 27Al NMR chemical shifts for AlH4−, AlH63−, and polymeric (AlH3)n are observed at 91 to 116, −46 to −31, and −18 ppm, respectively, and can be influenced by the nature of the counterion to some extent [47]. Five-coordinate aluminum hydrides have provided intermediate 27Al shifts ranging from 49 to 74 ppm in bridged metal hydrides [3]. However, the five-coordinate aluminum hydride environments in aluminum borohydride compounds are characterized by a much lower field 27Al signals up to 130 ppm [3]. 2.2. Scalar couplings Scalar (J) couplings or indirect spin–spin couplings are interactions between nuclear spins transmitted through bonding electrons. This is an important parameter for high resolution NMR as it provides direct information about atomic connectivity and thus about the three-dimensional molecular structure. Spin–spin couplings involving

27

Al are in general difficult to observe directly due to the dominating quadrupolar

interaction. By minimizing quadrupolar relaxation (reducing EFG fluctuations and/or increasing tumbling

molecular motions) and reducing dynamic exchange, favorable conditions can be reached, allowing observation of characteristic peak splitting. This is often met in nonpolar (non-interacting) organic solvents at low temperatures. In recent years there have been growing number of studies utilizing J coupling in correlation experiments in the solid state and novel methodologies are developed to measure this parameter in solids [111]. The coupling constant ranges for several pairs of nuclei involving 27Al are gathered in Table 3. Scalar couplings across up to three bonds have been measured and the only examples known are with protons. 2J couplings were observed with 1H, 11B, 13C, 29Si, and 31P nuclei. Other nuclei provided one-bond J coupling including 2H, 14/15N, 19F, 35Cl, and 81Br.

Table 3

In solution, scalar couplings (SC) can be determined through relaxation measurements and this method constitutes an alternative way to evaluate the scalar J coupling when a signal splitting is not directly observed in the 1D NMR spectra for different reasons like chemical exchange or quadrupolar broadening (T1,2 < 1/2πJ). Scalar relaxation requires the spin–spin coupling of a nucleus I to another nucleus S. In the case of a fastrelaxing quadrupolar nucleus S such as 27Al, the fluctuating magnetic fields at I arise under the influence of variations of spin state of S and the relaxation rate can be written as follows:

R  I

2

SC

 1/ T2 I 

SC

  4 / 3 (J IS )2 SS  1 (T1S  T2S / [1  (1 – 2 ) 2 T2S  ]) 2

where T2I, T1S, and T2S are the corresponding relaxation times (spin–spin and spin–lattice) of the nuclei I and S. The value of JIS can therefore be obtained from a plot of R2I as a function of (T2S, T1S). Changes in relaxation behavior can be achieved by varying the temperature [117].

2.3. Nuclear spin relaxation and quadrupole parameters Relaxation behavior and line-shape are also important parameters, which give indications about physical and chemical information, in particular dynamics and local symmetry. In general, the dominant relaxation pathway for quadrupolar nuclei such as 27Al is the quadrupolar relaxation mechanism, a process involving an interaction between the nuclear quadrupole moment and fluctuating EFGs at the nucleus. The EFGs originate from valence electrons directly involved in bonding to the element of interest, but also from others present in its vicinity. Under isotropic conditions (liquids), molecular tumbling modulates the interaction between the EFG and the nucleus, providing an avenue for nuclear relaxation. In this case the decays of the longitudinal and

transverse magnetizations of the quadrupolar nucleus are described by the sum of I + 1/2 (for half-integer I) exponentials, corresponding to allowed single-quantum transitions (m = ±1) between the 2I + 1 nucleus Zeeman energy levels. For I = 5/2 nuclei such as 27Al, the observed magnetization can be attributed to three components, that is, the central transition (mI = 1/2 -> −1/2) and the outer transitions (mI = 3/2 -> 1/2 and mI = −1/2 -> −3/2) and (mI = 5/2 -> 3/2 and mI = −3/2 -> −5/2). The T1 and T2 relaxation times for each component are critically dependent on the tumbling rate (correlation time c) and frequency ( 0) of the nucleus in question. Different situations are then possible and have to be considered separately. In case of extreme narrowing conditions (0c ≪ 1), where small molecules tumble rapidly in solution, the relaxation rates of all components are identical and quadrupolar relaxation is characterized by a single exponential decay. The general expression for the relaxation times T1 and T2 for any I > ½ nucleus is given by:

1/ T1  1/ T2   ½  (32 /10)  2I  3 /  I 2  2I  1  (Cq 2 c )(1   2 / 3)

with Cq  e2Qqzz / h     and        qxx – q yy / qzz ,  qxx  q yy  qzz where qxx, qyy, qzz are the principal components of the second-rank tensor of EFG, Q the quadrupole moment, Cq is the quadrupole coupling constant and  is the asymmetry parameter. The quadrupole coupling constant (Cq) is a key parameter that provides a measure of the relative symmetry of the electrons environment around the nucleus, while the asymmetry parameter () accounts for the deviation from cylindrical symmetry of the EFG at the nucleus, and ranges from 0 to 1. Note that under these conditions the signal linewidth (½) is directly proportional to c, and quadrupolar relaxation data give a product of Cq and c. Thus, Cq can only be calculated if c is determined by some other method. At the opposite extreme (0c ≫ 1), where large molecules evolve in the limit of slow isotropic molecular motion, quadrupolar relaxation theory predicts that the central transition can give rise to a relatively narrow signal, while the peaks due to all other components are broadened beyond detectability. For half-integer quadrupolar nuclei the linewidth for the central transition in this molecular motion regime can be written as follows:

½)mI = ½ -> -½ ∝ Cq2/(c02) where 0 is the resonance frequency of the nucleus. Notice that the linewidth of this signal decreases with increasing c, and that it is dependent on the resonance frequency 0 of the nucleus, which in turn is dependent on the external magnetic field B0. Here again, the linewidth is proportional to the square of the

quadrupole coupling constant, which reflects the magnitude of the coupling between the nuclear quadrupole moment eQ and the effective local EFG eqzz at the nucleus. Hence, a highly symmetrical environment around the nucleus will lead to a small EFG, and thus, a small value of Cq. It appears thus that the linewidth of a quadrupolar nucleus in the limit of slow isotropic motion is dependent on a number of crucial parameters including intrinsic nuclear properties (I, Q, 0), molecular properties (eq, c), and an empirical variable (B0). A second important property of the signal due to the central transition in the slow motion limit is that its chemical shift is also field dependent. This is known as the quadrupole-induced shift, and is an up-field shift whose magnitude decreases with increasing B0. This quantity can be written as follows:

 = (obs iso)mI = ½ -> -½ ∝ (Cq/0)2 where obs and iso are the observed and isotropic (in absence of second-order effects) chemical shifts respectively. This suggests that for half-integer nuclei under these conditions an increase of B0 translates into down-field shift in the signal corresponding to the central transition. This expression together with that for linewidth are field dependent, that is, ()mI = ½ -> −½ = f(0) and ( ½)mI = ½ -> −½ = f(0), and allow extraction of physical information (Cq, iso, and c) for a quadrupolar nucleus bound to very large molecule (such that 0c ≫ 1). 2.4. Quantification and “NMR invisible” aluminum Quantification in liquid-state NMR can be obtained directly from the intensity of the NMR signal by spin counting with respect to an internal or external reference. However, quantification of quadrupolar spins is often not straightforward, in particular in case of signals with large Cq, even though the optimal excitation conditions are established [118]. The fractional intensity fmI = n -> n+1 for the central and satellite transitions (mI = n -> n + 1; n = −I to I−1) for any quadrupolar nucleus at small enough pulse duration t were found to be approximately equivalent under a given amplitude of the radiofrequency (rf) field rf whether selective or not [118]:

f mI  nn1  t   { I  I   1   – n  n   1  / n I to I 1  I  I   1   – n  n   1}rf t In the case of 27Al nucleus (I = 5/2), and for the central transition we obtain:

f mI  ½½  t    9 / 35 rf t

Hence, in the slow motion limit one should theoretically observe only ca 26% of the signal efficiently, compared with conditions in which all components (satellite transitions) are detected (i.e., extreme narrowing). This means only the spin population associated with on-resonance excitation by the short pulse of length t behaves like a linear system. In this case, the response of the spin population is proportional to the excitation and independent of quadrupolar coupling strength. The relative line intensity of this on resonance transition therefore becomes quantitatively measurable only when a short rf pulse is used. This is also valid for solids where only the central line of half-integer spins is usually detected [119-121]. Even using small pulse angles, some fraction of the aluminum known to be present in well-defined solids (usually zeolites) remains unobserved; this is the so-called “invisible aluminum” [122-124]. Comparison can be made to a standard compound (e.g., α-Al2O3 [125]) to see how much signal is being lost. Differing T2* leads to differential signal loss during the initial system recovery time which can be compensated by backward linear prediction for the first points of the free induction decay (FID). However, very broad spectral components can be completely lost. This is due to 27Al NMR signals associated with aluminum sites with very low symmetry and, consequently, very large EFGs that produce broad NMR resonances. The question of “invisible aluminum” has been a common problem noted in the literature for solid-state 27Al NMR studies of alumina dehydration [58], pyrolysis of ceramic materials [42], and post-synthesis modification of zeolites [123, 124, 126, 127] that result in severe distortion of the aluminum sites. However both the theoretical understanding and the available experimental technology have improved markedly over the last few years. A combination of suitable experimental conditions, together with higher magnetic fields and faster MAS rates has removed many of the difficulties that gave rise to the idea of “NMR invisible” aluminum [128]. Alemany and coworkers [129] demonstrated that the distorted octahedral sites in zoisite [orthorhombic Ca 2Al3Si3O12(OH)] could be observed in 27Al MAS and 27Al MQMAS experiments at 11.7 T and higher magnetic fields using MAS speeds over 30 kHz. Similarly, Fyfe and coworkers [124, 130] have shown that 27Al MQMAS NMR at very high magnetic field (e.g., 18.8 T) allows

27

Al species to be measured in ultrastable-Y zeolite (USY), formed by steam treatment and

partial dealumination of zeolite HY. In addition, transfer of populations in double-resonance (TRAPDOR) NMR introduced by Grey and Vega [131] was applied to distinguish the protons that are coupled or not with 27Al and determine the quadrupole coupling constant of the “invisible aluminum” in dehydrated zeolite HY [123, 132]. “MAS NMR invisible” aluminum could be made visible by applying static

27

Al NMR sequences such as

quadrupole echo pulse [83]. Such advanced techniques (e.g., MQMAS, TRAPDOR, static echo), especially at high magnetic field strengths, show promise for elucidating the nature of distorted aluminum environments in zeolites, including previously “NMR invisible” quadrupolar species and structures of amorphous solids.

In solution NMR, the problem of undetectable aluminum may happen for two distinct reasons: (i) large Cq values for lowly symmetrical sites as in solids and (ii) slow correlation times for large molecules or species evolving in viscous media. For example, the colloidal aluminum hydroxide phase quantified indirectly from 27Al NMR spectra covers a broad hydrolysis ratio (h = [OH−]/Al3+]), 0.5 < h < 4.0 (at pH range 3.6 < pH < 13) [55]. At very low hydrolysis ratios (h < 0.5), where the formation of aluminum hydroxide was insignificant, the calculated total concentration of aluminum as a sum of all species quantified by NMR (Al monomers, “dimer,” Al13, and Al30) is very close to 100% of the nominal Al concentration. For higher values of h, NMR signal loss is usually observed and by subtracting the total “soluble” content of the sample from the pre-set nominal concentration of Al-ions, one can estimate the fraction of colloidal or insoluble Al species “invisible” in

27

Al

solution NMR spectra. The invisible parts of those 27Al NMR spectra sometimes make the peak base greatly wider, leading to severe distortion of their shape. Consequently, a lot of polymeric Al species may exist in solution but not be identified by experiments. These species seem to contain polymers assembled around a central tetrahedral aluminum [133], that could be viewed as resulting from aggregation of Al13 clusters. Such colloidal and/or precipitate [134, 135] may also contain oligomers with nuclearity in the range of Al2–Al12 [8]. In alkaline aluminosilicate solutions, the presence of condensed particles, in extreme cases giving rise to gels or precipitates, affects the 27Al NMR. At high aluminum concentrations, some large polymeric aluminosilicate species might exist. Such particles presumably give very broad signals, which are difficult or impossible to detect in high-resolution spectra, leading again to the phenomenon of “invisible aluminum” [77, 78]. The signals generally observed at pH values in the range 5–9 represent only a very small fraction of the aluminum content of the samples. Condensed particles increase in quantities as the pH is lowered to neutral values, but to unknown extents. Monitoring the unobserved quantities by conventional liquid state NMR would constitute a valuable in situ tool to measure solubility and to study aggregation, precipitation and crystallization phenomena.

3. Experimental and theoretical methods and techniques The complexity of NMR spectra of quadrupolar nuclei associated with the fact that the transposition of NMR techniques developed for 1/2-spins is not necessarily straightforward, probably explains why solid-state NMR of quadrupolar nuclei has taken longer to become a systematic characterization technique for aluminum-containing materials. However, after the initial development of MAS in the late 1950s, several routes have been proposed to deal with quadrupolar nuclei with broad CT, such as double-orientation rotation

(DOR) [136], quadrupolar magic angle turning (Q-MAT) [137] or two-dimensional experiments like dynamic angle spinning (DAS) [138], quadrupolar phase adjusted spinning sideband (Q-PASS). However, the major break-through technique, which has propelled NMR of quadrupolar nuclei in the solid state forward is the MQMAS experiment [139, 140]. The MQMAS experiment separates the isotropic part of the quadrupolar interaction in a second, indirect dimension, in which the individual resonances become resolved through their chemical isotropic quadrupolar shift. Further improvements have contributed to expand the range of application of this technique, such as satellite transition magic-angle spinning (STMAS) [141, 142], the combination of MQ/STMAS with cross-polarization (CP) or related experiments, as well as signal enhancement methods like double frequency sweeps (DFS), wideband uniform rate and smooth truncation (WURST), rotor assisted population transfer (RAPT), hyperbolic secant (HS), or Q-CPMG. The main features of the NMR experiments will be presented in the following sections, and we refer to the concept paper by Ashbrook and Duer for more detailed information about solid-state NMR of quadrupolar nuclei [143]. 3.1. Experiments under MAS In the case of compounds with multiple sites for a quadrupolar nucleus, each site has its own nutation frequency, which depends on the strength of the quadrupolar interaction. Therefore, acquisition of a quantitative NMR spectrum requires the use of a small flip angle  < /(2(2I+1)) associated with a short pulse duration (usually below 1 μs) to ensure the homogeneous excitation of the whole signals including their spinning sideband patterns. When the Cq is sufficiently large that features are visible on the central transition line shape, the quadrupolar parameters can be determined from lineshape fitting of the CT. When the Cq is too small, one has to consider the whole spinning sideband manifold of the so-called satellite transition spectroscopy (SATRAS) [144] NMR spectrum, since its spread and shape are characteristic of the q and  values, as illustrated for aluminum fluorides [96, 97, 145]. An alternative to the simulation of the whole spinning sideband manifold, is the generation of an “infinite spinning frequency” isotropic spectrum, using the two-dimensional one-pulse (TOP) procedure from a one pulse NMR spectrum [146, 147]. This allows the direct quantification of the various sites, as was shown for example for

27

Al sites in a series of aluminum

hydrofluorides [59, 61-63]. Finally, in the case of strong resonance overlap, one has to go toward the use of more advanced multiple pulse techniques such as MQMAS. The high-resolution NMR experiment MQMAS (Fig. 3) has greatly extended the use of NMR of halfinteger quadrupolar spins [148], as it is technically much more simple to implement than the DAS or DOR methods. Under MAS conditions, the MQMAS and related experiments (STMAS, slow CTMAS) use an echo

involving the symmetrical multiple-quantum transitions (−n/2 → n/2) of a quadrupolar nucleus to correlate in a 2D spectrum an isotropic indirect vertical dimension F1 with the powder pattern corresponding to the CTs in the direct horizontal dimension F2. Free from anisotropic line broadening, the projection of the resonances onto the F1 dimension is sharp and well resolved. Along the horizontal dimension, the features arising from the second order of the quadrupolar interaction remain, and the quadrupolar parameters can therefore be determined for each site resolved in F1. If the −3/2 → 3/2 transition is the most used (3QMAS), for quadrupolar nuclei with nuclear spin I > 3/2, the −5/2 → 5/2 transition can also be used (5QMAS) but at the price of a significant loss of efficiency. The MQMAS technique is now routinely used to characterize and study aluminum containing materials such as zeolites [30, 149] aluminophosphates [150-153] and MOFs [154].

Figure 3

3.2. Distance measurements and correlation experiments 3.2.1. Measuring J couplings in solids Indirect spin–spin couplings are hardly observable in solid-state in general, and particularly not with quadrupolar nuclei, of which measurements are hindered by quadrupolar couplings that are five orders of magnitude stronger than J-coupling constants. However, there are exceptions, and splitting patterns caused by scalar (J) coupling involving

27

31

Al have been detected in a few cases. For instance, the

P NMR of the

.

Me3P AlCl3 complex adsorbed on zeolite NaX under MAS exhibits a sextet pattern as a result of J-coupling to the six Zeeman levels of 27Al [155]. By using J-resolved experiments, it is also possible to measure the twobond coupling between 27Al and 31P nuclei in aluminophosphate materials [156]. Other indirect methods have been used to determine 31P–27Al J-coupling constants, like insensitive nuclei enhanced by polarization transfer (INEPT) and rotational echo double resonance (REDOR) based experiments. In the case of

27

Al ->

31

P INEPT

experiments, Kao and Grey used the standard INEPT sequence (Fig. 4a) [157]. The intensity of the INEPT response as a function of , the rotor-synchronized interpulse period, is given by the equation:

I ( )  Csin(2J Al  P ) exp(2 / T2 )

where C is a constant, T2 is the spin–spin relaxation time, and JAl–P is the two-bond 31P–27Al J-coupling constant. The J-coupling constant is deduced graphically from the plot IINEPT = f(). The technique REDOR is generally dedicated to isolated pairs of spin-1/2 and can be adapted to SI spin system, where S spin is a quadrupolar nucleus, by using a frequency selective (FS) approach [158, 159]. Scalar couplings JSI between 27Al and 31P in aluminophosphate VPI-5 have been determined by this method. The FS-REDOR sequence is shown in Fig. 4b as follows. Two experiments for each integer number of rotor periods (mTR), one with and one without applying 31

P pulse, are usually measured to record the resulted echo signal Se(2mTR) and the reference signal Sr(2mTR),

respectively. The normalized difference between the two signals can be expressed as:

R(2mTR) = (Sr – Se)/Sr = A{1 – cos[(2mTR - t)JAl-P] x cos[(2mTR - t)JAl-P]} where the scaling factor A describes the signal attenuation due to incomplete reversal of selective

31

P spin

magnetization, and t is a delay resulting from slower nutation of this magnetization during the selective pulse. Here again, the plot of R = f(2mTR) allows the extraction of the coupling constant JAl–P graphically.

Figure 4

3.2.2. Correlation experiments The initial solid-state experiment used to study correlation between pair of atoms is CPMAS. Under MAS condition, magnetization transfer from a spin-1/2 to a quadrupolar nucleus can indeed be achieved through the dipolar interaction in a CP experiment. Although the CPMAS experiment between 1/2 spins is now routinely used, CPMAS involving quadrupolar nuclei is more difficult to implement because (i) the Hartmann– Hahn matching condition is modulated not only by the RF and MAS frequencies, but also by the quadrupolar frequency and (ii) the efficiency of the spin-lock of the quadrupolar nucleus in the transverse plane is not trivial. CP transfer can nonetheless be effective using a fast MAS rate associated with low rf field on the quadrupolar nuclei. Traditionally, CP is performed from the highly abundant and fast-relaxing 1H nucleus to the

27

Al nucleus. The advantages of using CP are twofold: to boost the signal intensity of the quadrupolar

nucleus (because the gyromagnetic ratio of 1H is greater than that of 27Al) and to detect which atoms are close. CP is not restricted to the 1H–27Al spin pair, and examples of 27Al CP with 19F, 31P, 29Si have been reported.

The first heteronuclear correlation experiments, involving at least one quadrupolar nucleus, have been carried out based on the recoupling of the dipolar interaction averaged out under MAS conditions. The 2D cross-polarization heteronuclear correlation (CP-HETCOR) experiment however suffers from similar constraints to those encountered in the CP experiment, that is, the difficulty to efficiently spin-lock the quadrupolar nucleus. The heteronuclear dipolar recoupling is also sensitive to the carrier frequency offset, which may become a problem when spectra are recorded at high static magnetic field. An alternative, proposed by Amoureux and coworkers, consists of using the heteronuclear multiple quantum correlation (HMQC) pulse sequence associated with a dipolar recoupling pulse sequence (D-HMQC) [160]: the magnetization on the quadrupolar nucleus is refocused by the selective echo, while the double-quantum (DQ) coherence is excited by application of recoupling pulses on the spin 1/2 nucleus, reducing the sensitivity of the experiment to carrier frequency offset. For example, surface hydroxyl groups in γ-alumina were characterized by

27

Al

MQMAS and 1H–27Al 2D NMR spectroscopy. In particular, the terminal versus bridging character of the hydroxyl groups observed in the 1H MAS NMR spectrum was distinguished from 1H–-Al distance measurement and latter assigned to the corresponding aluminum neighbor by analysis of {1H}–27Al 2D NMR spectrum [161]. In organoaluminum compounds, an additional difficulty to the measurement of 13C–27Al distances by NMR arises from the close Larmor frequencies of the two nuclei, making the tuning of common NMR probe heads impossible [162]. This has been circumvented by the use of diplexers that provide tuning to two nuclei with close Larmor frequencies, with the limitation that both nuclei cannot be irradiated simultaneously. NMR methods have been developed to measure

13

C–27Al distances and establish connectivity maps in organo-

aluminum and MOF materials [163]. Despite being usually hidden by the heterogeneous line broadening, the scalar part of the J-coupling interaction can also be used to generate high-resolution correlation NMR spectra, in particular under fast MAS and high-field conditions. The intrinsic difficulty of dealing with a quadrupolar nucleus in J-based NMR experiments can be overcome by using “soft” pulses, selective for the CT of the quadrupolar nucleus. This way, the quadrupolar nucleus behaves like a “fictitious” spin 1/2 nucleus and polarization transfer through a JHMQC or a J-HSQC experiment is feasible. The J-coupling constant values between a spin-1/2 and a quadrupolar nucleus can also be measured using J-resolved pulse sequences, as was shown for example for the 31P–27Al spin pair in aluminophosphates [115, 156, 158]. Homonuclear correlations (HOMCOR) between half-integer nuclei can be probed using the DQ for dipolar-coupled-quadrupolar nuclei (DQ-DCQ) or D-HOMCOR pulse sequences, which are HORROR-like

experiments based on rotary resonance recoupling and associated with the use of weak CT-selective radiofrequency pulses. In this experiment, the DQ coherence from a single quadrupolar spin is suppressed while that arising from dipolar-coupled nuclei is retained. The 2D NMR spectrum obtained contains correlation peaks between dipolar coupled quadrupolar nuclei (separated by internuclear distances below 5.5 Å), as was shown for the 27Al–27Al spin pair in aluminophosphate AlPO-14 [164-166]. 3.2.3. Distance measurements Double resonance experiments are designed to determine heteronuclear internuclear distances between dipolar-coupled nuclei. They rely on the measurement of the signal loss of a I-spin upon recoupling of the I-S dipolar interaction (partially averaged out by MAS). For a given recoupling time, the normalized magnitude of the signal loss depends on the strength of the dipole–dipole coupling constant. The initial experiment in this group, REDOR, was introduced by Gullion. It is however not particularly well suited when a quadrupolar nucleus is involved, and therefore more sophisticated recoupling processes have been proposed like the transfer of populations in double resonance (TRAPDOR), the rotational echo adiabatic passage double resonance (REAPDOR), the transferred echo double resonance (TEDOR), the rotational-echo saturation-pulse double-resonance (RESPDOR), etc. Estimation of distances between quadrupolar nuclei of similar nature can be obtained under MAS conditions using symmetry-based recoupling schemes like HORROR. Provided there is prior knowledge of the dipolar and/or quadrupolar interactions, the analysis of the measured DQ filtered curves delivers information on the magnitudes and relative orientations of dipolar and quadrupolar tensors, as was shown for 27Al in the molecular sieve aluminophosphate AlPO [164-166]. 3.3. High magnetic field As previously mentioned, an additional difficulty for quadrupolar nuclei arises from the fielddependent shift and the line broadening generated by the second-order perturbation of quadrupolar interactions under the dominant Zeeman interaction. If MAS improves the resolution, it does not completely remove the anisotropic broadening if the quadrupolar interaction is large. As the quadrupolar interaction is inversely proportional to the square of the Larmor frequency, a quadratic gain both in resolution and sensitivity is expected at higher magnetic fields. The explosion of very high-field NMR solid spectrometers (up to 1.1 GHz to date) has therefore greatly participated to the use of 27Al NMR spectroscopy for a very wide

range of compounds, crystalline or amorphous, inorganic or organic, by providing NMR spectra of very high resolution. 3.4. DNP measurements The sensitivity of solid-state NMR spectroscopy is an intrinsic problem that directly translates to very low signal-to-noise NMR spectra. The recent development of dynamic nuclear polarization (DNP), which relies on electron polarization rather than on nucleus polarization, under MAS conditions and high magnetic field [167], has greatly enhanced the NMR sensitivity for solids, making possible the detection of low concentration species in a reasonable amount of time [168]. The major application of MAS-DNP involving the 27Al nucleus concerns the characterization of surface species in catalysts. For example, studies reporting the characterization of the surface of γ-alumina nanoparticles by 27Al DNP-NMR were reported, in which the local symmetries of the aluminum sites at the surface of the particles could be determined [169]. Another application concerns porous materials, through which the radical used to create the hyperpolarization can diffuse. For example, Pourpoint and coworkers used MAS-DNP to measure

13

C–27Al distances in aluminum-

based MOFs [163]. 3.5. Modeling and theoretical calculations With the rapid development of computational chemistry, its application has become almost ubiquitous in all branches of chemistry. Because NMR properties are very sensitive to atomic positions, their calculation using DFT-based codes have proven reliable, when coupled with experimental NMR parameter determination, to improve and assess the accuracy of structural data. Computational modeling, along other experimental methods, is often used to complement 27Al NMR experiments to help signal assignment and to understand NMR parameters with respect to the structural and chemical properties. In the study of Al(III) compounds, computational methods have been used to predict NMR parameters including chemical shielding (CS) [74, 105, 114, 170-174], indirect scalar couplings [114, 175], and quadrupolar coupling parameters [34, 114, 171, 173, 176]. For infinite periodic compounds, DFT-based codes includes WIEN2K [177, 178] (for the EFG calculation) and the codes based on the GIPAW (Gauge Including Projector Augmented Wave) method, like CASTEP (Cambridge Sequential Total Energy Package) [32, 179], for calculations of the chemical shift tensors were developed and have provided great help in the assignment and understanding the spectra of a broad variety of solids [16, 17, 180]. 3.5.1. Chemical shielding calculation

27

Al NMR

An increasing number of publications about ab initio calculations of

27

Al CS tensors has appeared in the

recent literature [74, 171, 172, 174, 181-185], with some of them reporting 27Al CS tensors [105, 173, 186]. Comparison of experimentally determined and theoretically calculated CS tensors provides information on the orientations of these tensors in the molecular frame as well as acting as a rigorous test of modern computational methods. For example, Schurko and coworkers [173] reported theoretical 27Al CS tensors using restricted Hartree–Fock (RHF) methods and DFT for some Al(III) and Al(I) model solid compounds. Mirzaei and coworkers [105] used DFT to calculate isotropic and anisotropic

27

Al CS parameters in the investigation of

models of aluminum phosphide nanotubes, although no experimental data were available. In general, the calculated isotropic shifts are slightly higher than the measured chemical shifts. Wang and coworkers [174] systematically checked the accuracy of the popular theoretical models for computing the 27Al chemical shifts by comparing the calculated and experimental chemical shifts in more than one hundred aluminum(III) complexes. Despite the systematic difference between the absolute calculated and the experimentally observed values, the general trend obtained is usually helpful to assign the spectra, interpret the experimental data, and understand the structure-property relationships. For instance, Dedecek and coworkers used DFT and molecular mechanics (MM) calculations to investigate the effect of Al/Si substitutions and the presence of silanol nets on the

27

Al NMR CSs as well as the local tetrahedral AlO4− of the nearest and next-nearest

neighboring Al atoms in silicone-rich zeolites [172]. More recently, Florian and coworkers also investigated the Al/Si ordering in the crystalline gehlenite Ca2Al2SiO7 by means of 27Al NMR and ab initio quantum mechanical calculations, enabling the identification of the seven aluminum sites due to the Al/Si substitutions despite their strongly overlapping NMR lines [114]. By using quantum mechanics DFT, Xu and coworkres calculated the CS of the Al atoms present in different fragments of AlPO-11 framework and compared these to experimentally observed chemical shifts during the different stages of the crystallization process, allowing identification of the initial structural building units [170]. Theoretical and experimental 27Al NMR chemical shift studies were also performed on aluminate species in solution such as aluminosilicates [74]. Some of the observed differences in theoretical data of a set of aluminosilicate oligomers were not previously detected experimentally due to the lack of spectral resolution. Therefore, in such a situation, theoretical calculations of NMR chemical shifts can be very helpful and can provide some missing information. 3.5.2. J-coupling constant calculation J couplings are generally difficult to calculate with high accuracy using quantum chemical methods, in large part because of the pronounced dependence of the coupling operators on the description of the electron density at the nucleus. Bryce computed J couplings involving 27Al with 17O and 31P nuclei occurring in some

model compounds [36]. The results showed that the coupling constants 1J(27Al, 1

27

dominated by the Fermi contact contribution. The calculated values for J( Al, 2

27

17

17

O) and 2J(27Al,

31

P) are

O) were moderately low

31

ranging from 0 to 38 Hz, whereas those of J( Al, P) were also in the same range from 0 to 32 Hz. Although no available experimental data exist for 1J(27Al, 17O), the experimental values for 2J(27Al, 31P) (Table 3) compare quite well with the calculated ones. Bond lengths and bond angles have the most significant influence on the coupling constants. 1J(27Al, 17O) increases with increasing Al–O bond length and 2J(27Al, 31P) increases with Al– O–P bond angle, in agreement with the proposal of Massiot and coworkers [156]. Also, in a recent contribution, computed scalar couplings 2J(27Al, 29Si) and 2J(27Al, 27Al) have been found to be linearly related to the Al–O–Si and Al–O–Al bond angles respectively [114]. Calculated values of 2J(27Al, 29Si) were found between 2 and 5 Hz, slightly larger than the experimental values, which lie within the range 1–4 Hz. For the Al–O–Al bonding schemes, values of 1.0–3.5 Hz were calculated for 2J(27Al,

27

Al), which had been never measured

experimentally. The 2J(27Al, T), where T = 27Al or 29Si, were indeed found to be linearly correlated with the Al– O–T bond angle . The slope of this correlation J/ = 0.3–0.5 Hz/deg was significantly smaller than the 3.4 Hz/deg found for Si–O–Si in calcium silicates but is also outside the 130–170° bond angle range of purely silicate materials. These calculations show that the scalar coupling is a local probe of the atomic environments through the bond angle. 3.5.3. EFG and quadrupolar parameters calculations Because they are quadrupolar (I = 5/2),

27

Al nuclei interact with EFGs created by surrounding charges,

leading to so-called quadrupolar broadening. From the analysis of this signal broadening it is possible to extract the quadrupolar coupling constant (CQ) and asymmetry parameter () values, which characterize the local environment of the probed quadrupolar nucleus. DFT calculations are known for their ability to estimate molecular structures, and therefore EFGs, accurately. The quadrupolar parameters, that is, Cq = e2Qqzz/h, and

 = ∣qxx − qyy∣/qzz, can thus easily be calculated from the principal components qii of the EFG tensor. The consistency obtained between experimental and theoretical determination of EFG tensors is usually sufficiently good that helps to achieve a better understanding of the NMR data with respect to the structural and chemical properties to the system under investigation. For example, Schurko and coworkers showed that the relative CS and EFG orientations along with the orientations of the largest components of the EFG tensors in the molecular frame of some representative Al complexes can be predicted theoretically [173]. Recently, calculation of the 27Al tensors in the ZnAlF5∙[TAZ] [176], a MOF-type materials with 1,2,4 triazole (TAZ) ligand, have been run on both the powder XRD and DFT-optimized structures. The NMR parameters (Cq and )

calculated for the optimized structure were closer to those determined experimentally than the values calculated from the powder XRD structure, highlighting the improved atomic positions after optimization. DFT geometry optimizations have also been carried out in order to estimate Cq and  values of Al3+ sites in aluminum substituted indium sulfides (In1−xAlx)2S3, and thus to help signal assignment [34]. Based on direct comparison of the magnitude of the experimental and calculated values, assignment of the experimental signals to the different possible sites in the structural models was proposed. The combination of experimental 27

Al NMR and DFT calculations has then been shown to be a powerful strategy for precise understanding of the

local structure around the Al3+ ion in partially occupied/substituted sites. The use of DFT calculations of EFG was also reported recently to study Al/Si ordering in minerals [114]. It has been shown that random structures systematically overestimate Cq with respect to the experimental values, while the ordered structure provides underestimated value. However, structures with Loewenstein-rule violating pairs give values for Cq very close to those from experiment. Calculation of EFGs is also important in inorganic fluoride and hydroxyfluoride materials to support crystal structures determined from X-ray data, and to characterize the potential distribution of F/OH species in the coordination spheres of aluminum cations. The 27Al EFG is indeed particularly sensitive to the local geometry of the 27Al polyhedron. For structures determined from powder XRD data, the localization of the oxygen or fluorine atoms can be approximate, leading to a strong discrepancy between calculated and experimental 27Al NMR parameters. A step of structure optimization at the DFT level, which reduces the forces acting between atoms, usually solves the issue. In aluminum-based MOFs,

27

Al NMR is very sensitive to the nature of the

aluminum inorganic node (cluster, chains sharing opposite corners, edges, etc.). In the EFG calculations, care has to be taken when hydroxyl groups are present, as the resulting 27Al EFG becomes highly sensitive to the hydration rate, due to hydrogen-bonding which modifies Al–O distances. It is therefore very important to consider in the DFT-calculations and in the experiments compounds with strictly identical hydration rates [187].

4. Examples of applications 27

Al NMR spectroscopy to investigate the nature of aluminum compounds and their behavior in both

solution and solid states still has an important place among the available analytic tools, especially with the increasing development in NMR methodology and instrumentation during the past few years, which have made measurement and data analysis much easier. Thus, there is now renewed interest in some old topics

where fundamental questions persisted, since modern spectroscopy offers the potential to reach new insights. The examples that will be presented in this section cover some representative studies of aluminum chemistry. When possible, both solid and liquid-state are presented. The reports illustrate the NMR spectroscopic speciation in the controlled hydrolysis of aluminum solutions, an introduction of aluminum in polyoxometalates (POMs) and their characterization by 27Al NMR, and the study of the crystallization process of microporous materials such as aluminosilicate zeolites, aluminophosphate zeotypes, and aluminum carboxylate MOFs. 4.1. Aluminum(III) cation in aqueous solution To date, only a few aluminum polycation structures have been unequivocally identified by

27

Al NMR in

solution namely the Keggin isomers -Al13, -Al13, and -Al13 and the 30-mer Al30 [50, 52, 53, 188]. Although other aluminate species were suggested to exist in partially hydrolyzed solutions, their firm characterization by

27

Al NMR remains questionable and assignment of their spectra still needs structure confirmation.

Combination of 27Al MAS NMR and XRD data of isolated polycationic species would constitute an interesting approach for species/sites identification in solution by 27Al NMR [50, 52, 53, 188-190]. 4.1.1. Relationship between 27Al chemical shifts and Al–O distance in AlO4 environments The

27

Al chemical shift of aluminum polycations is mainly influenced by structural parameters like

coordination number and Al–O distance, which affect the electronic density distribution around the aluminum atom. For a given coordination number, the Al–O distances modulate the observed 27Al chemical shift, and the 27

Al chemical shift thus provides a way to access to the average bond length in a coordination

polyhedral AlOn. For example, the case of a tetrahedral environment, which is usually more symmetrical than the octahedral environment and in turn easier to determine, is illustrated in Fig. 5. The observed chemical shift moves to higher fields with the increase of the distances.

Figure 5

With the relationship versus (27Al) shown in Fig. 5, one can predict the chemical shift of an AlO4 site from a given structure or estimate Al–O distance in a tetrahedral environment knowing its 27Al chemical

shift. This relation reflects the electronegativity of the oxide ligands (O2−) polarizing the Al–O bonds, which induce a deshielding effect on the 27Al nucleus with a magnitude depending on the Al–O distances. The shorter these distances, the less the electronic density around the Al center, and higher the frequency of the

27

Al

resonance. 4.1.2. Conversion of -Al13 into Al30 Allouche and Taulelle [54] demonstrated by using in situ 27Al NMR experiments under hydrothermal and room temperature conditions that aluminum monomers are the species controlling the -Al13 Keggin conversion into Al30. New 27Al NMR signatures of Keggin-type aluminum polycations were also observed in solution during the process. A chemical pathway was proposed to explain the isomerization during the condensation reaction. The ε-Al13 polycation is the principal product of hydrolysis, at h = nbase/nAl = 2.5, of a Al(NO3)3 aqueous solution by NaOH at 90 °C with vigorous stirring. The final pH of the solution is typically ca 4. The 27Al NMR spectrum of the resulting solution (Fig. 6a) shows the characteristic peaks of ε-Al13 (63 ppm) and monomers (0 ppm). When this solution is heated between 50 and 150 °C or aged during 6–24 months at 20 °C, ε-Al13 transforms into new aluminum polycations, especially Al 30.

Figure 6

The

27

Al NMR spectrum of a solution containing -Al13 and monomers heated at 130 °C, during 15 h,

presents five signals in the AlO4 chemical shift range: 63, 64.5, 70, 76, and 81 ppm (Fig. 6b). The -Al13 (63 ppm), -Al13 (76 ppm), and Al30 (70 ppm) species were unambiguously assigned [53, 189]. The signal at 64.5 ppm could correspond to Keggin aluminum polycations having a central AlO 4 in a shell of AlO6 [192]. A new signal, of very low intensity, appears at 81 ppm. At 80 ppm one finds the Al(OH) 4− signal, but this species does not exist at pH 3.7. The signal at 81 ppm must, therefore, be assigned to another Keggin polycation. Temperature is not essential to produce Al30. At 20 °C with aging, a Al30 solution spontaneously evolves to a solution containing -Al13 and monomers (Fig. 6c). As the 27Al NMR signal of Al30 (70 ppm) is broader than 1500

Hz, its detection at room temperature is difficult. Increasing the temperature accelerates the conversion and enhances the 27Al NMR observation by narrowing the 27Al line. The kinetics of -Al13 conversion was followed by periodically recording the 27Al NMR spectrum, at 120 and 85 °C. The evolution of the signal intensities with time is consistent with a scheme of sequentially appearing species [50]. In the early stages of -Al13 (63 ppm) conversion into Al30 (70 ppm), a weak signal at 64.5 ppm is present. This species is probably very similar to -Al13, due to the close similarity of its chemical shift to that of -Al13. Aluminum monomers (0 ppm) are also produced during the -Al13 transformation. Following the thermal treatment, two new species arise, with signals at 76 and 81 ppm. At 85 °C, the 76 ppm signal, due to -Al13, does not appear before 6 h. At both 120 and 85 °C the 81 ppm signal, though qualitatively detected, stays below 1%. It is possible to obtain by direct synthesis, a pure -Al13 solution, with h = 2.46, [Al] = 0.5 M and T = 100 °C. The quantitative analysis of the

27

Al NMR spectrum confirms that -Al13 is the only species present in the

sample and that no monomers are present. This solution heated at 130 °C during 1 h, monitored by NMR in situ, does not exhibit any chemical transformation of -Al13. The temperature narrows the line width of the 11 ppm band, assigned to the 12 AlO6 sites of -Al13, but aluminum monomers represent <2% of the total aluminum present. The -Al13 cation can therefore be thermally decomposed in monomers, but very slowly. However, only 5 min after the appearance of aluminum monomers, Al30 (70 ppm) and other Keggin polycations (64.5 and 76 ppm) appear in solution [192]. The same behavior is observed when aluminum monomers are deliberately added to the pure solution of -Al13. The presence of aluminum monomers in solution is critical to promote the synthesis of Al30, and they initiate the first step of -Al13 conversion into Al30. Based on these observations, a chemical pathway is suggested, sketched in Fig. 7, to rationalize the transformation of -Al13 and the appearance of other aluminum polycations. Initially, -Al13 is supposed to be “capped” by a monomer to form an intermediate species ε-Al14, which could be responsible for the 64.5 ppm NMR signal. The NMR data recorded at 85 °C show that the concentration of -Al14 (from signal intensity of 64.5 ppm resonance) is low and remains almost constant. As -Al13 decreases and Al30 increases, one may safely assume that -Al14 is formed from ε-Al13 during the first stage of transformation and consumed by formation of Al30. This will lead for a period to a stationary, small amount of -Al14. The -Al14 species would isomerize into -Al14 and would further dimerize and react with two other monomers to produce Al 30. The monomer by capping -Al13 shares three μ3-hydroxo bridges with a trimer sub-unit of -Al13. This addition to -

Al13 imposes some structural distortions on the Keggin structure, stretching of Al–O bounds, facilitating the 60° rotation of the trimer sub-unit. Recently, it has been shown that fluorination of -Al13 leads to partial substitution of di-2-OH bridges by fluorine and enhances the formation of Al 30 [193].

Figure 7

After Al30 formation, two other aluminum polycations signals appear successively in solution, at 76 and 81 ppm. The former is the -Al13 Keggin isomer and the second most probably another Keggin form. The 27Al NMR chemical shift of aluminum sites is sensitive to the electronic density around the 27Al nucleus. It is a function of the coordination number of aluminum, the chemical nature of the first coordination sphere, and the angles and distances of chemical bonds. For an AlO4 inside a Keggin unit, the chemical shift is therefore mainly a function of the average distance Al–O. Knowing the 27Al NMR assignment of the AlO4 sites of -Al13 (63 ppm), Al30 (70 ppm), and -Al13 (76 ppm), a systematic difference of +7 to 6 ppm appears between the -, -, and Keggin forms. As the chemical shift differences observed for the different species, 63, 70, 76, and 81 ppm, present an increment of 6 ± 1 ppm, it is possible to infer the signals at 81 ppm to -Al13. The -Al13 has unfortunately not been characterized by NMR [52]. One can assume that its chemical shift must be coincident with Al30 for the same reasons as above. This is in agreement with the recent theoretical calculations showing the progressive increasing order in stability for Keggin aluminum from - to -isomer [194]. 4.2. Aluminum in polyoxometallate compounds POMs have been of particular interest in the fields of catalytic chemistry, surface science, and materials science because their chemical properties such as redox potentials, acidities, and solubility in various media can be finely tuned by choosing appropriate constituent elements and countercations. In particular the coordination of metal ions to the vacant sites of lacunary POMs is one of the most effective techniques used for constructing efficient and well-defined active metal centers. This is of considerable importance considering the relationship between the properties and activities of these centers and the local structure of these sites. Such synthetic methodology offers a new opportunity for the syntheses of purely inorganic aluminum based compounds with structurally well-defined aluminum sites; in contrast, little success has been achieved in controlling the aqueous aluminum chemistry involved in the hydrolysis process. However, despite these

limitations in the case of hydrolysis, various types of aluminum-coordinated POMs, either alumino-molybdates [195-200] or –tungstates [10, 24, 201-204], have been isolated. The structural aspects and 27Al spectroscopic characterization of some of these will be discussed in the following sections. The Anderson-type structure is one of the most familiar aluminum polyoxomolybdates, Al(OH)6Mo6O183− [195], and consists of six edge-sharing MoO6 octahedra surrounding a central AlO6. The structure is planar with D3d symmetry. The six OH groups are linked to the central Al octahedron stabilizing the compound. The

27

Al

liquid-state NMR of the polyanion is characterized by a relatively narrow signal (line width = 100–250 Hz) at +15 (± 0.5) ppm (Fig. 8) consistent with a highly symmetrical octahedrally coordinated Al. This signal appears quite deshielded by comparison to resonances of aluminum molybdate Al 2(MoO4)3 showing four peaks at negative  values between −10 and −15 ppm [205]. Aluminum molybdates are compounds of interest for catalysis. They are often believed to form in Mo-containing hydrotreating catalysts utilizing Al2O3 or zeolite supports.

27

Al MAS NMR has become established as a tool of choice to identify the nature of the active

aluminum molybdate species. In 1993, Edwars and Decanio [197] observed a new resonance at 13 ppm in their

27

Al MAS NMR spectra of rehydrated calcined Mo/Al 2O3 hydrotreating catalyst precursors. They

attributed

this

resonance

to

the

presence

of

hydrated

forms

of

aluminum

molybdates

[Al(OH)n(H2O)6−n]n(MoO4) (n = 1 or 2). Some years later, it was demonstrated that in fact this resonance corresponds to an Anderson-type aluminomolybdate complex deposited at the surface of the support [196, 201]. Moreover, the signal of crystalline Al 2(MoO4)3 formed on ZSM-5 zeolite have been found to occur at −15 ppm but only after high temperature treatment [198, 200]. Interestingly, rehydration leads to disappearance of this signal and simultaneous appearance of the 13 ppm resonance previously assigned to Anderson-type POM anion, although the authors assigned it to octahedral Al in MoO 3∙Al2O3∙nH2O, n being the coordination number of H2O, n > 1 [25]. This demonstrates the high stability of the Anderson-type POM anion and its ability to form readily from the mineral aluminum molybdate Al 2(MoO4)3 by simple hydration when the latter is dispersed on a porous support. Such a transformation can be described by the following reaction:

4Al2  MoO4 3  15H2O  2Al  OH 6 Mo6O183  3Al2O3  6H3O In a recent paper [199], the sequential thermal transformation from the Anderson-type aluminum molybdate to the mineral Al2(MoO4)3 by heating at 200–800 °C and further to alumina by heating at higher temperatures (800–950 °C) is consistent with the above sequential formation of aluminum molybdate species observed on supported catalysts first as Anderson-type structure and then as Al2(MoO4)3 upon calcination.

Figure 8

While aluminomolybdate POMs are known to occur only as Anderson-type structures, aluminotungstates showed more POM structural diversity including the most common structures like Keggin [24, 201-203], Dawson [204], and their derivatives [206, 207]. The aluminum cation can be integrated into these compounds either by occupying the internal cavities as a heteroatom [24, 201-203], to substitute the tungsten site or more likely to coordinate the lacunary site of the structure [24, 201, 202, 204]. This offers opportunities for diverse synthetic strategies as well as structural and chemical variety. Also, Son and coworkers [208, 209] showed that combination of aluminum based polycations along with tungstate polyoxoanions can lead to nanocluster composite materials with novel properties. The clusters bound to each other by electrostatic interaction and via hydrogen bonding. It has been observed that the Al30 polycation exchanges metals in the composite with the H2W12O426− (W12) polyoxoanion to form a new compound, W2Al28, that is nearly isostructural with the Al30 structure. The new compound differs in that a WO6 group replaces one of the capping AlO6 groups at each end of the Al30 cation. This supports the proposed mechanism of Al30 formation as being governed by Keggin-type aggregation promoted by monomers as proposed in Fig. 7. The potential of aluminum chemistry in POM compounds appears therefore very promising despite the fact little efforts have been devoted to this field. Among the earliest reported aluminum based polytungstates, one can cite the Keggin structures and their derivatives such as - and -isomers AlW12O405− (AlW12), the lacunary - and -isomers AlW11O409− (AlW11), and the Al- and V-monosubstituted derivatives Al(AlOH2)W11O396− (AlAlW11) and AlVVW11O406− (AlVW11) respectively [24].

27

Al NMR enabled their differentiation and quantification as well as monitoring kinetics of their

transformations. The 27Al NMR spectrum of -AlAlW11 exhibits two distinct resonances at 73 ppm (½ 98 Hz) for the central tetrahedrally coordinated Al site and 8 ppm (½ 237 Hz) for the octahedrally coordinated Al site. The spectrum of -AlW12 showed a unique narrow resonance at 72.1 ± 0.1 ppm (½ 1–2 Hz) while the signature of the -AlW12 isomer was 71.7 ± 0.1 ppm (½ 5–6 Hz). In the spectrum of -AlVW11 the resonance appeared at 72.5 ppm (½ 220 Hz). The sodium salt of -AlW11 is characterized by a signal at 66.9 ppm (½ 783 Hz) whereas the isomer -AlW11 by a signal at 66.7 ppm (½ 702 Hz). A substantial difference is observed

in the potassium salt of -AlW11 showing a resonance at 63.3 ppm and a line width of ½ 1735 Hz. These variations, probably due to ion pairing, reflect the dependence on the nature and concentration of the counteraction present. Other conditions like Keggin concentration and total ionic strength could also affect the

27

Al NMR parameters. By controlling all these experimental conditions, it was nevertheless possible to

monitor detailed isomerization paths in AlAlW11, its transformation into AlW12, and further its controlled hydrolysis to various isomers of AlW11 [202]. This allowed preparation of novel mixed metal aluminotungstates with general formulae AlMn+W11O39(9−n)−, Mn+ = Mn(II/III/IV) or Co(II/III), and some of these were characterized by 27Al NMR. In a more recent study [203], the AlW12O405−/6− homogeneous self-exchange rate constant was measured by

27

Al line broadening. This was possible due to the fact that the two separate aqueous

diamagnetic AlW125− and paramagnetic AlW126− ions showed sharp resonances (½ < 1 Hz) at 72.9 and 74.7 ppm respectively. Other Al containing Keggin type structures, such as the vacant Al OhW11, the lacunary vacant AlOhW10, and the lacunary Al-monosubstituted AlTdAlOhW10, had also been reported and characterized by means of 27Al liquid NMR [201]. The spectrum of the vacant AlOhW11 showed one resonance in the octahedral chemical shift at ca 11–12 ppm, while the lacunary vacant AlOhW10 is characterized by a higher field signal at around 7 ppm. The lacunary Al-monosubstituted AlTdAlOhW10 presented two resonances as expected for tetrahedrally Al site at ca 73–74 ppm and another at ca 6 ppm for the octahedral Al site. Recently, the syntheses, molecular structure and NMR characterization of two Dawson-type trialuminumsubstituted polyoxotungstates have been reported [204]. The monomeric Dawson-type structure H3P2W15O59[Al(OH2)]36− (P2W15Al3) is obtained in acidic pH, while the hydrogen-bonded dimeric form {H3P2W15O59[Al(OH)]2Al(OH2)}216− is stabilized at basic pH. The 27Al NMR spectra showed contrasted chemical shifts. While the monomeric Dawson structure is characterized by a signal at ca 1 ppm, the dimer showed a much broader resonance around 22 ppm. Such a deshielding effect appears consistent with the strong hydrogen bonding interaction involving the hydroxyl groups attached to the aluminum trimers. As in the Keggin structure types, the deshielding effect with respect to the free Al 3+ monomers (0 ppm) suggests that the aluminum ions should be coordinated to the trilacunary site. By comparison, signals were observed at 18 ppm for the zirconocene derivative of the monoaluminum-substituted phosphorotungstate Keggin PW11O39Al(OH)ZrCp26− (Cp = C5H5−) [207], and in the range 8–13 ppm for the polyoxoanion PW9O34{W(OH)(OH2)}{Al(OH(OH2)}{Al(-OH)(OH2)2}27− [206]. The situation is different in the case of silicotungstate where the Al is found to be present as countercation (extra-framework atom) as proved by its narrow 27Al line at 0 ppm [210].

4.3. Aluminosilicates and zeolites 4.3.1. Speciation in zeolite synthesis precursors The 27Al NMR spectra of sodium aluminate dissolved in H2O shows a narrow peak at 80 ppm, indicating the presence of the tetrahedral Al(OH)4− species. Al(OH)4− is the only species of any significance in alkaline (pH = 7– 14) aluminate aqueous solution Alkaline aluminosilicate solutions have extensively been investigated by 27Al NMR spectroscopy. Although there has been some confusion in the literature regarding the assignment of

27

Al chemical shifts [211], it is

now commonly accepted that the shifts at 80, 75, 70, 65, and 60 ppm are ascribed to the species q0, q1, q2, q3, and q4 respectively [79], where qn represents Al(OH)4−n(OSi)n, as depicted in Fig. 1. In addition to the directneighbor oxygen atom, the nuclei of the second coordination sphere contribute to the shielding of the fourcoordinated aluminum and thus to the chemical shift in the 27Al NMR spectrum. For instance, increasing the Si/Al ratio in the aluminosilicate solution increases the intensities of the more shielded resonance. This change of the chemical shift in the NMR spectrum is due to the fact that more Al–O–Si bonds are created and thus that the incorporation of a Si atom corresponds to an increase of the shielding of the Al nucleus. Harris and coworkers [79] studied the evolution with time of an aluminosilicate solution to gain some insights into the incorporation process of aluminum. The different spectra specified some aspects of the incorporation process. Next to the Al(OH)4− species, there were already some q2 and q3 aluminum species present in solution. During the process, the peaks at the highest shifts decreased in intensity, whereas the intensity of the q2 and q3 peaks increased. Only after a longer period of mixing, the peak at 60 ppm (q4 species) became visible but the evolution of the incorporation process was not yet finished after 16 h of mixing. Here, the rearrangement of q0 and q1 species is too fast for the NMR time scale to resolve their separate chemical shifts. However, the peak at 80 ppm observed at the beginning of the process slowly changed in the chemical shift toward 75 ppm and this is a clear indication that the equilibrium shifts to convert the q0 species into q1 species. Overall, it is stated that the distribution of the aluminum atoms gradually changes from small oligomer species toward q3 and q4 species. Some loss of total

27

Al NMR intensity is observed, which could

indicate that relatively large particles are formed [79]. Two-dimensional

27

Al exchange spectroscopy (EXSY)

NMR spectroscopy has also been employed to study the exchange processes in the aluminosilicate ions [79]. The observed strong cross-peaks between q1 and q2 resonances indicated the dominant exchange between those two species within the timescale of the NMR experiment. In contrast, the EXSY experiment illustrated that the q3 and q4 species are not involved in an exchange with other species. Therefore, the q3 and q4 species

seemed to be more stable than q1 and q2 and this could be explained by the fact that species involved in more than two siloxane bridges are stabilized with much reduced bond opening/closing frequency. NMR experiments can give information about the coordination number, connectivity, and concentration of soluble aluminate and silicate species occurring in zeolite precursors [212]. Shi and coworkers [213] performed in situ solid-state NMR study for the crystallization of zeolite A. The 29Si and 27Al NMR spectra showed broad resonances, indicating the formation of a network of SiO4 and AlO4 tetrahedra. The induction period did not provoke large changes in the NMR spectra, suggesting that this method was not sensitive enough to detect the changes of the gel in the initial synthesis stages. In a similar study, Miladinovic and coworkers [76] monitored the crystallization of zeolite A by in situ liquid state

27

Al NMR. A sharp signal at 79 ppm was observed due to the dissolved Al(OH) 4− species. The

concentration of this Al species and hence the intensity of the

27

Al NMR signal decreased during the

crystallization and could be plotted as an S-shaped crystallization curve suggesting a mechanism via direct consumption of dissolved Al(OH)4− monomers. On the other hand, the intensities of the signal at 59 ppm, representing the tetrahedral Al(OSi)4 building blocks, increased over time. In a combined 29Si and 27Al NMR and small-angle X-ray scattering (SAXS) study, Eilertsen and coworkers [72] examined the hydrolysis of tetraethylorthosilicate (TEOS) by N,N,N-trimethyl-1-adamantammonium hydroxide (TMAdaOH) in a clear sol precursor of a high-silica CHA zeolite (SSZ-13). By analyzing the initial stages of this hydrolysis process, they quantitatively characterized the silicon atoms in the dissolved oligomers and in the nanoparticles, as well as their Si connectivity. During hydrolysis of TEOS, the average Si connectivity increased and at a [Si]/[TMAdaOH] ratio of one, nanoparticles were detected. Similarly, the connectivity of the aluminum atoms was investigated and similar trend to that of the silicon atoms was observed. Where q 0 and q1 were found to be the principal species in the beginning, q2, q3, and q4 gradually came to predominate during the hydrolysis process (Fig. 9). When the [Si]/[TMAdaOH] ratio is one, hence at the point where nanoparticles are formed, all the aluminum atoms are tetrahedrally coordinated to four silicate units and located into the nanoparticles.

Figure 9

27

Al NMR spectra can be used to probe the relative effects of Li +, K+, and Na+ on the incorporation of Al into

the aluminosilicate solutions. Clear trends can be deduced from the NMR spectra in Fig. 10 showing the example of Na and K in the system tetramethylorthosilicate (TMOS):tetramethylammonium hydroxide (TMAOH):Al(iOPr)3:H2O:NaCl:KCl. The different resonance peaks broaden with an increasing amount of inorganic cations, indicating that there is a more pronounced chemical exchange or, rather, that condensation reactions take place, leading to larger aluminosilicate species. This latter process is also revealed quantitatively, as the amount of non-observable Al-species increases up to 50–60% when adding extra Li, Na and K cations. Condensed aluminosilicates gradually develop and become too large to be observed by

27

Al

liquid NMR.

Figure 10

In the spectra shown in Fig. 10, some resolved resonances could be distinguished, in particular in the q2 and q3 ranges. The signals for q0 (80 ppm) and q1 (75 ppm) are assigned to the monomers Al(OH)4− and to chainending singly connected Al respectively. The q2 range shows two main signals at 69 and 71 ppm that can be assigned to Al doubly connected to silicates either in linear or four-ring cyclic environments. In the q3 range, at least three resonances can be seen at 67 ppm for one of them and ca 65 ppm for the two others with one exhibiting particularly narrow line. As for q2, it would be possible that q3 species are also differentiated whether the Al is located in linear or four-ring cyclic environments. This would correspond to the two broad lines. Interestingly, the narrow line should present a highly symmetrical environment around the Al atom averaging out almost to zero the EFG. Such a configuration would be impossible with three connections to silicate moieties. However, with four siloxane connection types, a highly symmetrical Al environment becomes possible. One can therefore suggest a spiro-like bicyclic structure for such symmetrical aluminosilicate, O7Si2AlSi2O7 (see insert in Fig. 10). The presence of the Al in strained three-membered rings would explain the particularly deshielded resonance for such q4 species. This is a well known phenomenon in

29

Si NMR of

27

silicates and the same could happen as well for the Al NMR of aluminosilicates. The causes of line broadening of the resonance peaks in the

27

Al of aluminosilicate could be multiple,

including local structural distortion, slow dynamic tumbling, and increase in chemical exchange rates. These exchange processes may be investigated by two-dimensional

27

Al EXSY NMR. Fig. 11 shows

27

Al EXSY NMR

spectra of the sample 0.6 TMOS:0.8 TMAOH:0.5 Al(iOPr)3:100 H2O for different mixing times. First, strong cross peaks appear between the q0 and q1 Al-specie resonances (80 and 75 ppm), indicating an exchange between the corresponding Al sites simply by connecting/disconnecting Al monomers to existing (alumino)silicate species. This is the most probable process with only one-step bond creation/breaking energetically favored for mobile end group sites. The same bond formation/rupture involving more condensed species would be less favorable considering the stabilization effect of a more elaborated connected network. With increasing mixing time, correlations appear between resonances q0 and q2 as well as between some q2 species. Based on the assignment of the two resonances of q2 type as being due to Al species in middle linear chain and four-ring cycles, these exchange peaks reflect cyclization process, where the simplest example of the four-ring oligomer formation is illustrated in Fig. 11. The opening/closing of tetrameric aluminosilicate species implies exchange between q1 and q2(cyclic) in one hand and between q 2(linear) and q2(cyclic). While this latter is observed in the spectrum, the former can however hardly be seen because the q1 Al is widely involved in the exchange with the q0 monomers, and instead, exchange correlations between q0 and q2 sites are obtained.

Figure 11

4.3.2. 27Al Solid-State NMR of zeolites The presence of aluminum in framework sites of zeolites generates a negative charge that can be compensated by protons producing bridging Si–OH–Al hydroxyl groups, which are Brønsted acid sites. The identification, quantification and localization of the aluminum atoms present in either framework or extraframework species (EFAL) in the zeolitic framework is the main information that can be extracted from solid-state 27Al and 29Si NMR spectroscopy [214]. The relative intensity of the signals of Si(nAl) environments in the 29Si MAS NMR spectra of aluminosilicate zeolites gives direct information on the second neighbouring atoms (Si, Al) of silicon sites and their relative populations. It is particularly interesting to determine the Si–Al ratio in aluminum rich zeolites. The assignment of specific crystallographic sites or groups of sites in zeolite can be performed by applying the empirical expression (Al) = −0.50  + 132 (ppm) [215]. This expression correlates the 27Al isotropic shift with structural parameters of the AlO4 tetrahedron, in this case with the average T–O–T angle (). Comparison with the mean

T–O–T angle calculated for every crystallographic site of the zeolite structure resolved by XRD allows the assignment of every 27Al signal to a site or a group of sites. Thermal/hydrothermal treatment of zeolites usually releases aluminum atoms from the framework, giving rise to EFAL species. This is often accompanied by the appearance of Lewis acidity, which has also been associated with the occurrence of three-coordinated framework aluminum [216]. Recently, the application of 2D 1H and 27Al DQMAS NMR techniques in combination with theoretical calculations has shown the proximity of Brønsted/Lewis acid sites in dealuminated zeolites, and has provided better understanding of the structureactivity relationship [217]. Well-defined EFAL species can be introduced by chemical vapor deposition (CVD) methods, as for example shown by Pydko and coworkers, who explored the deposition of trimethylaluminum for the preparation of model faujasite-type catalysts containing extraframework aluminum. The state of intrazeolite Al atoms, the changes in zeolite structure and acidity caused by the CVD procedure as well as by subsequent treatment were analyzed in detail by 1H,

29

Si, and

27

Al MAS NMR. Such zeolites display much

higher activity per Brønsted acid site in propane cracking than does a commercial ultrastabilized Y zeolite. It was proposed that the activity enhancement is related to strong polarization of a fraction of the zeolite Brønsted acid sites by Lewis acid sites formed by the hydrogenolysis of grafted trimethylaluminum complexes [218]. Aluminum site changes upon dehydration/rehydration of zeolites can be monitored by

27

Al NMR

spectroscopy, as recently shown for H-Beta zeolite [219]. It has also been shown, by quantitative 27Al MAS NMR analysis combined with 1H MAS NMR, that some active EFAL species formed during calcination can reinsert into the framework during this hydration process. The presence of AlO6 is usually associated with EFAL species. However, the reversible formation of octahedral species linked to the framework has been observed for zeolite beta, ZSM-5 and zeolite Y. The reversibility of this transformation is proven by the 27Al NMR spectrum, which contains two sets of resonances (one for the AlO6 and one for the AlO4): upon transformation, the relative ratio between the two resonances varies but the overall integral remains the same [220]. 4.4. Aluminophosphate zeotypes 4.4.1. Hydrothermal NMR to study formation mechanisms of oxyfluorinated aluminophosphates

Understanding the crystallization mechanisms of nanoporous solids remains one of the most challenging issues in materials science. Taulelle developed the in situ NMR methodology under hydrothermal conditions to investigate the structure, dynamics, and stability/reactivity of the soluble species present in the synthesis medium during the crystallization [26, 54, 221-226]. This section describes how the formal secondary building units (SBU) concept for solid construction can experimentally be investigated, checked and validated on some representative porous phosphates. The role of reactive species identified in solution is discussed with respect to their evolution into more elaborate intermediate structures such as the prenucleation building units (PNBU) or the neutral molecular building units (MBU). In some cases, the proposed models could not be generalized, depending on the reaction conditions, the chemistry of the metallic cation, and the stability/solubility of the target phase. Specific experimental equipment and devices therefor need to be developed, and adapted both to the NMR experiments and the chemical system. In situ NMR techniques were used to monitor the evolution of the reaction from the introduction of the precursors to the precipitation of the final solid in real time and under various conditions of temperature, pressure, and time (T, P, and t) [226-228]. Taulelle and coworkers therefore designed a new setup (Fig. 12) allowing such observations [26, 224, 229, 230] and fulfilling seven severe requirements. This setup must (i) withstand up to 200 °C, with a safety margin of up to 250 °C, (ii) withstand pressures up to 50 bars, (safety margin up to 200 bars), (iii) be inert to reagents (HF, H3PO4, Al(OH)3, organic ligands and amines of different natures as well as any combination of these), (iv) be resistant to chemical aggressiveness of very basic as well as very acidic conditions (pH< 1 or >13), (v) be almost transparent to radio-frequency, (vi) have diamagnetic properties to avoid a severe perturbation of the B0 field homogeneity (11.7 T for high resolution), and (vii) avoid any use of aluminum in the probehead [26]. 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 °C [223]).

Figure 12

This new experiment has the unique advantage to provide, beside the value of the true pH at 200 °C, identification and quantification of the different species, versus P, T, and t, and their temporal evolution in the

liquid. These in situ NMR investigations of the solution part constitute a complement to ex situ solid state NMR [21, 111] and in situ XRD [231-234] studies of the solid part. Three solids, obtained by hydrothermal synthesis at 180 °C, were studied: (NH 4)[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 most the atoms present in these structures are common NMR nuclei. By using specific in situ hydrothermal devices, NMR experiments allowed the evolution of the signal of each atom during the reaction to be followed and therefore provided independent information about each particle during the reaction. The 3D structure of AlPO4 CJ2 [235, 236] can be described (Fig. 13a) as tetramers containing two PO4 groups corner-linked to two Al polyhedra in six and fivefold coordination respectively. One fluorine atom is shared between the two Al polyhedra, while the second, located on the Al octahedron, is terminal. Ammonium ions are inserted in the tunnels.

Figure 13

The NMR spectra were recorded in the range 30–210 °C for each of the types of NMR nuclei present in the structure, and of these the most informative nucleus was 27Al (Fig. 13b–d). The ex situ study concerned the mother liquor above the solid (Fig. 13c), whereas in situ was dedicated to the solid–liquid mixture (Fig. 13b). The time-evolution of the signals showed several steps in the formation of the SBU. At 30 °C, the Al monophosphate is the only species (i) present (Fig. 13c). Al3+ ions are octahedrally surrounded by one phosphate oxygen, two OH/F groups, and three H 2O molecules. When temperature increases, a change in the Al coordination first occurs with a loss of one water molecule leading to the species (ii) in Fig. 13c. During the rest of the reaction, fivefold 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, as depicted in Fig. 13b (species/signal iii). This species was formed only in presence of excess non-dissolved reagent (in heterogeneous medium), presumably when supersaturation occurs. Indeed, when the synthesis liquor is filtered out and measured as a homogeneous liquid sample by NMR, only the initial species (i and ii) are observed changing their coordination environment reversibly when increasing or

decreasing the temperature. Finally, this oligomeric condensation is followed by an intramolecular nucleophilic addition establishing Al–F–Al bridges by rotation of the Al-F bond of one of the two aluminum atoms. As a result, this latter undergoes an increase in its coordination number from five to six. This results in the SBU observed in the solid as shown by its 27Al MAS spectrum in Fig. 13e. The isomerization correlated to the Al–F–Al bridge seems to occur in the solid state during crystallization, because in the in situ dissolution experiment (Fig. 13d), the spectrum resulted in a unique resonance at ca 55 ppm, which falls in the chemical shift range of species (iii). This means the tetrameric SBU complex is not stable as in its isolated form in solution, and it undergoes a slight structural rearrangement with breaking the strained bridging bond. ULM-3 [237] and ULM-4 [238], synthesized using two different templates, also exhibit a 3D structure (Fig. 14) with Al or Ga cation. In this case, they are both built up from vertice connections of equivalent hexamers Al3P3 (central insert of Fig. 14) instead of the previous tetramers Al2P2. The two external aluminum-centered polyhedra exhibit fivefold coordination, whereas the central one is sixfold. However, their SBU connection is different as shown on Fig. 14a. Their formation, investigated using the same procedure as above, proved to be similar to the formation 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.

Figure 14

4.4.2. Solid-state NMR of AlPOs 27

Al NMR is the tool of choice for the study of microporous aluminophosphates (AlPOs) [14, 239-246].

As mentioned earlier, the coordination-state of the cations can indeed be determined, as the

27

Al NMR

isotropic chemical shift is characteristic of the 27Al coordination state: tetracoordinated AlIV resonates around 50 ppm, pentacoordinated AlV around 0 ppm and hexacoordinated AlVI around −20 ppm. Potential interruption of the framework (Al–OH or P–OH terminal bonds) can be detected from 2D 1H–31P or 1H–27Al NMR experiments, this being essential information needed to determine the topology of the inorganic framework. As mentioned earlier, the establishment of

27

Al–31P connectivities from high-resolution NMR data is very

helpful to drive the search for a structural model from powder diffraction pattern. By reducing the complexity

of the system and the number of independent parameters, even very complicated structures could be solved from powder diffraction data [20]. Because the literature is very rich on this topic, we refer to Ref. [14] for further examples. Solid-state NMR is also powerful for investigating further the distribution of fluorine atoms in the framework of fluorinated AlPOs, which are not accessible from diffraction data (powder and single-crystal). In particular, the nature of the distribution (random or not) of the iso-electronic species F− and OH− can be evaluated by recording 19F–13P and/or 19F–27Al 2D NMR spectra: in ULM-3(Al), all bridging positions between the aluminum polyhedra were found fully occupied by F− ions while in AlPO4-CJ2 and ULM-4(Al) a random distribution of F− ions and OH groups has been clearly identified [247, 248]. When several positions are available for fluorine, NMR can also provide information about its location. For example, in the microporous AlPO4-CLO solid (CLO = cloverite-type structure), F can either be trapped in double-four ring (D4R) cages forming the framework, or be positioned at the terminal positions (Al–F, P–F), interrupting the framework. 2D correlation 19F–31P, 19F–27Al, and 19F–19F NMR experiments are consistent with the presence of one F ion within the D4R unit and hydroxyl groups on the terminal positions of the aluminum and phosphorus polyhedra [249]. Phase transformations potentially occurring upon calcination and/or dehydration of microporous aluminophosphates can be monitored by solid-state NMR. For example, AlPO-53 was shown to transform into JDF-2 upon dehydration. The reversibility of the dehydration/rehydration process can also be studied. For example, in AlPO-(Al5P7)-DAE and AlPO-(Al5P7)-DAP (DAE = diaminoethane, DAP = diaminopropane), heating at 150 °C under vacuum leads to removal of water molecules and bridging hydroxyl groups [150, 250]. In AlPO(Al5P7)-DAP, no structural transition is observed and leaving the sample under air, the water molecules reenter the framework at the same place as in the “as-synthesized” solid. In contrast contrary, for AlPO-(Al5P7)DAE, which crystallizes in a polar space group, removal of water molecules and OH groups is accompanied by a phase transition. Analysis of the 27Al–31P spatial proximities in a high-resolution 2D NMR spectrum (Fig. 15), has allowed identification of the new—less symmetrical but also polar—space group and of the final topology of the solid. When leaving the sample under air, both species re-enter the framework, but the water molecules are now located randomly in the framework, as evidenced by highly broadened 31P and 27Al NMR resonances. This is in fact in agreement with the polar character of the space group: it would have been very surprising that had a rehydration process spontaneously regenerated a polar order of the layers.

Figure 15

Knowledge of dynamics of both the organic templates and the framework in AlPOs is important, but difficult to access at the atomic level, as XRD data are usually uninformative. It was shown that the presence of molecular dynamics on the microsecond timescale induced a modulation of the quadrupolar interaction, which could be detected by comparing the linewidth in the isotropic dimension of MQMAS and STMAS NMR spectra. It was for example illustrated in the case of the AlPO-14 compound, in both its as-synthesized (templates and water still present) and calcined-dehydrated forms, which shows significant line broadening in the

27

Al STMAS NMR spectra. In these compounds, the dynamic aspect was further confirmed by variable

temperature NMR measurements, and by recording of

27

Al dipolar-quantum filtered (DQF) ST-MAS NMR

spectra. Dynamics were shown to be due to both the templates and the water molecules in this solid [239]. 4.5. Metal-Organic Frameworks (MOFs) 4.5.1. Formation mechanisms of MOF type aluminum trimesates Despite the popularity of porous hybrid materials, the rationale behind their formation remains a mystery. The Al trimesates (1,3,5–benzene tricarboxylates) are typical systems within which several compounds can exist by varying the chemical conditions of the synthesis. Indeed, three phases, with completely different three-dimensional crystal structures, appear in this system 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-96 [251] combines linked corrugated chains forming hexagonal 18-membered ring tunnels, at the center of which appears the trimeric cluster. This trimer represents the primary building unit in the mesoporous MIL-100 [252], whereas MIL-110 [154] is built up from the association of Al8 octamers. At short times (<5 h), increasing pH successively leads to MIL-100 in a very narrow range of pH (0.5 < pH < 0.75) [253], MIL-96 (0.75 < pH < 3.25) [251] and MIL-110 (pH > 3.25) [154]. Above 60 h of reaction, the repartition has completely changed: MIL-100 has disappeared and MIL-110, which was formed in 4 h at pH 3.5, exists now only at very low pH (< 0.5), the MIL-96 domain being almost unchanged. This surprising behavior has been investigated by the combination of X-ray powder diffraction and in situ NMR method [225].

Four 27Al NMR 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 solutions. It is assigned to uncomplexed cation Al(H 2O)63+. The resonance at 1 ppm appears when the temperature is increased. Its presence is correlated with the presence of benzene1,3,5-tricarboxylate (btc) in solution, and is therefore assigned to the primary complex Al(H2O)5(H2btc)2+. This labile complex undergoes fast chemical exchange with Al(H 2O)63+. The signal at 4 ppm mainly arises from MIL110 formation; the other (at 7 ppm) is correlated to the formation of MIL-96 and, to a lesser extent, to the formation of MIL-100. Coming back to their structures, MIL-110 is based on original octamers, whereas MIL-96 and MIL-100 share different structural features, for instance the trimer Al3(3-O)(H2O)2(OH)(btc)2. Furthermore, as the 7 ppm contribution appears always latter than the 4 ppm signal, the two species should be structurally related. Therefore, the substructure Al2(2-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(2O)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 dimer complexes Al 2(2-OH)2(H2O)6(H2btc)3+ and Al2(2OH)(H2O)6(H2btc)23+ respectively. Assignments of 27Al signals are based on comparison between ex situ and in situ characterization of respectively solid and liquid parts along the synthesis (Fig. 16). According to Férey SBU concept [254], the species present in solution at the moment of crystallization are directly related with the building units of the crystal.

Figure 16

4.5.2. Solid-state 27Al NMR of MOFs In Al-based MOFs, the aluminum atoms always are in an octahedral environment, but can form a wide variety of entities, ranging from isolated aluminum octahedra, isolated aluminum clusters or infinite chains of octahedra sharing corners or edges. The 27Al 1D NMR spectrum provides a fingerprint of the Al-cluster [154, 252, 255-260]. 27Al MQMAS NMR provides further indication about the number, the relative intensity, and the shape of the NMR resonances that should be sensitive to the nature of the connectivity between the AlO6

octahedra (Fig. 17). For example, in the “open-pore” form of MIL-53(Al), a very characteristic lineshape is observed with large Cq about 9 MHz and  = 0, providing a clear signature of chains of AlO6 octahedra transconnected by corners [261]. The

27

Al NMR spectrum is also very sensitive to hydrogen bonding between

hydroxyl groups present on the Al atoms and water molecules in the pores, as can be seen by comparing the NMR spectra of MIL-53(Al) in its hydrated and dehydrated forms (Fig. 17, bottom spectra). It is therefore very important when DFT calculations are used that the calculations are performed on a crystal structure that has exactly the same hydration state as the measured compounds. Otherwise, strong mismatch in Cq—and potentially Q—values are observed [187].

Figure 17

5. Concluding remarks The use of 27Al NMR spectroscopy for the characterization of the structure and dynamics of a wide variety of compounds, in both the liquid and solid-state, has become popular nowadays, partly due by the ease of use of the technique. Both the acquisition of the 27Al NMR spectra and the tools for their interpretation, including ab initio calculations, have recently undergone considerable evolution, and are accessible to most chemistry labs. This is attested by the diversity of applications that we have reported in this chapter. In liquid state, 27Al NMR provides insights into structural, chemical, and dynamic properties of aluminum compounds, thanks to spectral parameters of the

27

Al nucleus such as chemical shifts, spin–spin couplings,

linewidths, and relaxation times. In solid-state, direct information about the aluminum coordination environment can be obtained from high-resolution NMR spectra. Further information about through-space and through-bond connectivity also provides invaluable information that can complement crystals structure determination using diffraction - a so-called NMR Crystallography approach. The diversity of compounds that can be investigated by

27

Al NMR spectroscopy, including aluminum

complexes, organoaluminums, soils, mineral solids, solid catalysts, zeolites, molecular sieves, porous compounds, and the important information that is derived from the NMR measurement are probably the major factors responsible for the growing popularity of the technique.

Acknowledgements University of Versailles Saint-Quentin en Yvelines and the CNRS are acknowledged for their financial support.

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Glossary of abbreviations AlPO: Aluminophosphate btc: Benzene-1,3,5-tricarboxylate CASTEP: Cambridge sequential total energy package CLO: Cloverite-type structure CP: Cross-polarization CS: Chemical shielding CT: Central transition CVD: Chemical vapor deposition D4R: Double-four ring DAE: Diaminoethane DAP: Diaminopropane DAS: Dynamic angle spinning DCQ: Dipolar-coupled-quadrupolar nuclei DFS: Double frequency sweeps DFT: Density functional theory DNP: Dynamic nuclear polarization DOR: Double-orientation rotation DQ: Double-quantum DQF: Dipolar-quantum filter EFAL: Extraframework aluminum EFG: Electric field gradient EXSY: Exchange spectroscopy FID: Free induction decay FS: Frequency selective GIPAW : Gauge including projector augmented wave HETCOR: Heteronuclear correlation HMQC: Heteronuclear multiple quantum correlation HOMCOR: Homonuclear correlation HS: Hyperbolic secant INEPT: Insensitive nuclei enhanced by polarization transfer MAS: Magic-angle spinning MBU: Molecular building unit MM: Molecular mechanics MOF: Metal-organic framework MQMAS: Multiple-quantum magic-angle spinning NMR: Nuclear magnetic resonance PNBU: Prenucleation building unit POM: Polyoxometalate Q-MAT: Quadrupolar magic angle turning Q-PASS: Quadrupolar phase adjusted spinning sideband RAPT: Rotor assisted population transfer REAPDOR: Rotational echo adiabatic passage double resonance REDOR: Rotational echo double resonance RESPDOR: Rotational-echo saturation-pulse double-resonance rf: Radiofrequency RHF: Restricted Hartree–Fock

SATRAS: Satellite transition spectroscopy SAXS: Small-angle X-ray scattering SBU: Secondary building unit SC: Scalar coupling ST: Satellite transition STMAS: Satellite transition magic-angle spinning TEDOR: Transferred echo double resonance TEOS: Tetraethylorthosilicate TMA: Tetramethylammonium TMAda: N,N,N-Trimethyl-1-adamantammonium TMOS: Tetramethylorthosilicate TOP: Two-dimensional one-pulse TRAPDOR: Transfer of populations in double-resonance USY: Ultrastable-Y zeolite WURST: Wideband uniform rate and smooth truncation XRD: X-ray diffraction

Fig. 1.

29

27

Si and

Al NMR spectra of alkaline silicate and aluminosilicate solutions emphasizing similarities between

n

resonances of Q silicates (SiO4−n(OSi)n) and qn aluminosilicates (AlO4−n(OSi)n). Fig. 2. 27Al NMR chemical shift range as a function of coordination number of aluminum (six or higher in green; five in orange; four in blue; three in purple) and as a function of ligand type. The coordination number in some cylopentadienylaluminum compounds may exceed six providing the highest field resonances [7]. These ranges apply for both liquid and solid-state 27Al NMR. 27

Fig. 3. Schematic of the various MQMAS pulse schemes for Al (I = 5/2) along with their coherence transfer pathways: (a) the basic two-pulse, (b) z-filter, (c) z-filter with split-t1, and (d) shifted-echo split-t1 sequences. The dotted pathway in case of (c) indicates the required coherence transfer for 3QMAS experiment while the solid pathway for the 5QMAS version. The evolution periods in the split-t1 sequences are defined as t1’ = (1/1+K)t1 and t1” = (K/1+K)t1 where K = 19/12 for the 3Q transfer and K = 25/12 for the 5Q transfer. In shifted-eco sequence (d),  is echo delay time. Fig. 4. Pulse sequences for (a) standard

27

Al ->

31

P INEPT and (b) FS-J-RES experiments employing selective excitation

pulses and rotor-synchronized evolution period (). Fig. 5. Correlation between observed

27

Al NMR chemical shift and average Al–O bond length in AlO4 environments of

aluminates. The interatomic Al–O distances are derived from XRD dada [52, 53, 188, 190], except for Al(OH)4−, which was from theoretical calculations [191]. Fig. 6. (a) Referenced 27Al NMR spectrum at 20 °C of a freshly hydrolyzed Al(NO3)3 aqueous solution (h = 2.46, pH = 3.7) containing monomers and ε-Al13. (b) Expansion of the AlO4 chemical shift range of a hydrolyzed solution (h = 2.46) after 12 h at 130 °C. (c) 27Al NMR spectrum of an aged (6 months) hydrolyzed solution (h = 2.46) containing initially monomers and ε-Al13. From Ref. [54]. Fig. 7. Proposed -Al13 isomerization initiated by monomeric aluminum hexaaquo cations to form Al 30. From Ref. [54]. Fig. 8. Stability domain for aluminomolybdate Anderson polyanion in aqueous solution as a function of pH: (a) 27Al NMR spectra showing the characteristic resonance at 16 ppm of the central Al in Al(OH)6Mo6O183− in the range 2 < pH < 6; (b) pH dependence of fraction of Al(III) as soluble Al3+, Al(OH)6Mo6O183−, and solid precipitate Al(OH)3 at C(Mo) = 0.2 M and 3+

2−

+

3−

C(Al) = 0.01 M. The models were computed using formation constant for “Al + 6 MoO4 + 6 H = Al(OH)6Mo6O18 ,” log K = 50.48, and solubility product for Al(OH)3, log Ks = −30.48. Fig. 9. Evolution of

27

Al NMR spectra of clear solutions/sols with the progress of TEOS hydrolysis by TMAdaOH in the

system 1–19 TEOS:0.5 Al2O3:7.5 TMAdaOH:438 H2O.

27

i

Fig. 10. Al NMR spectra of the system 0.6 TMOS:0.8 TMAOH:0.5 Al( OPr)3:100 H2O:x NaCl:y KCl: (x, y). (a) (0.0, 0.0), (b) (0.0, 0.1), (c) (0.05, 0.1), (d) (0.1, 0.2), (e) (0.1, 0.25), (f) (0.1, 0.3), (g) (0.2, 0.4).

Fig. 11. 2D 27Al NMR EXSY spectra of the system 0.6 TMOS:0.8 TMAOH:0.5 Al(iOPr)3:100 H2O, acquired with two distinct mixing times ( = 0.1 and 10 ms), and illustrative examples of the main exchange processes.

Fig. 12. Picture of the NMR tube and its components for hydrothermal conditions.

27

Fig. 13. (a) Projection of AlPO4 CJ2 with its tetrameric building unit; (b) in situ variable temperature liquid-state Al NMR 27

spectra of the synthesis precursor of AlPO4 CJ2; (c) variable temperature liquid-state Al NMR spectra of liquor synthesis of AlPO4 CJ2 after filtration from solid products/reagents; d) liquid-state 27Al NMR spectrum of dissolved AlPO4 CJ2 in water at 210 °C; and (e) solid-state 27Al MAS NMR spectrum of AlPO4 CJ2. The main fluorophosphate aluminate species observed in the synthesis solution are depicted. The 1:1 fluorinated Al monophosphate complex (i) adopts an octahedral coordination environment at room temperature prior to heating, as revealed by NMR. Upon heating, the Al monophosphate complex transforms to a fivefold coordination (ii) by losing one coordinating water molecule. Dimerization of the complexes leads to the terameric PNBU (iii), which is stable enough to exist in synthesis solution. After an intramolecular nucleophilic addition (yellow arrow) involving the rotation of an Al–F bond, the final Al2P2 SBU (iv) is obtained in the final solid only.

Fig. 14. (a) Structure of ULM-3 (left) and ULM-4 (right) constructed from the same hexameric Al 3P3 unit (middle) and (b) time resolved 27Al NMR spectra during their hydrothermal synthesis. Fig. 15. Left: Pulse sequence scheme for 27Al31P MQ-D-R-INEPT correlation. Middle: 2D 27Al31P MQ-D-R-INEPT NMR spectrum of AlPO4-(Al5P7)-DAE. Right: Representation of the building units determined from this NMR spectrum: P-O-AlO-P-O-Al square or linear chain, isolated PO4, and AlPO4 motif. Fig. 16. (a) Correlation between the time-evolution of aluminum species in the solid phase and the liquid phase, as 27

revealed respectively by ex situ XRD and in situ liquid state Al NMR at 180 °C in two separate syntheses: synthesis I and II optimized conditions for MIL-96 and MIL-110 respectively (synthesis I: 1.2 Al(NO3)3:1.0 Me3btc:500 H2O; synthesis II: 1.5 Al(NO3)3:1.0 Me3btc:250 H2O:4.5 HNO3). The darker each color, the stronger the intensity of the diffraction peak or the NMR signal. (b) correlation between the structure of species observed in the synthesis solution and the motif in the corresponding final crystals. The complexes observed in solution represent a part of the SBU of the final solid product. Fig. 17. 27Al MAS NMR spectra of a series of Al-MOFs. The connectivity of the AlO6 octahedra in the crystal structures is displayed in the right part of the Figure.

Table 1 27

Al NMR chemical shift range of aluminum center in a given local environment in terms of coordination number and

ligand type in the first coordination shell. Threecoordinate

Compound family

Fourcoordinate

Fivecoordinate

Sixcoordinate

Ref

Oxygen donors Phosphates/phosphonates(OP)

–

50 to 30

30 to 0

10 to −35

[9, 18-23]

Perchlorates (–OCl)

–

–

–

−20 to −25

[3]

Sulfates/sulfonates (–OS)

–

–

–

5 to −20

[3]

Nitrates (–ON)

–

90 to 80

–

5 to −20

[3]

Molybdates/tungstates (–OM)

–

75 to 60

–

20 to −15

[3, 11, 24, 25]

Aluminates (–OAl)

–

90 to 50

40 to 25

20 to −10

[9, 11, 26]

Carboxylates/alkoxides (–OC)

–

95 to 35

40 to 30

40 to −10

[3, 5, 27, 28]

Silicates/germanates (–OSi)

–

80 to 40

50 to 20

20 to −5

[9, 11, 12, 29, 30]

Borates (–OB)

–

90 to 55 Halides

45 to 30

15 to 0

[3, 31, 32]

Fluorides (–F)

–

85 to 35

50 to 20

5 to −15

[6, 26]

Chlorides (–Cl)

–

110 to 85

65 to 50

10 to −5

[3]

Bromides (–Br)

–

105 to 60

50 to 35

–

[3]

Iodides (–I)

–

50 to −30 Sulfure donors

–

–

[3]

Sulfides (–S)

–

125 to 100 – Nitrogen donors

25 to 15

[33, 34]

Cyanides (–N≡CX)

–

70 to 40

–

35 to −30

[3]

Pyridines (–NR=C)

–

155 to 95

75 to 65

30 to 0

[3, 35-38]

Amines (–NR–C/–NR2–C)

–

160 to 110

110 to 50

50 to −15

[3, 4, 39-41]

Nitrides (–N)

–

115 to 100 55 to 30 Phosphorus donors

Phosphines (–PR3)

–

160 to 90 75 to 50 Alkyls, hydrides, and silyls

Alkyls (–CR3)

280 to 220

225 to 40

Hydrides (–H)

–

Silyls (–SiR3)

–

a

10 to 0

[11, 42]

35 to 25

[43-45]

135 to 50

45 to −115a

[3, 7, 46]

165 to 90

155 to 45

110 to −45

[3, 47]

190 to 85

–

–

[3, 48]

The coordination number in some cylopentadienylaluminum compounds may exceed six providing very high field

resonances [7].

Table 2 27

Al Chemical shifts of tetrahedral Al sites in the solid state correlated to the electronegativity of the first coordination

sphere atom. Al environment (AlX4)

 range (ppm)

Electronegativity of atom X (χ)

AlO4

70 to 45

3.44

[11, 56-58]

AlN4

114

3.04

[35, 42]

AlS4

125 to 100

2.58

[33, 34, 102]

AlC4

120 to 111

2.55

[110]

AlP4

142

2.19

[106]

Ref

Table 3 J-coupling constant (Hz) ranges between 27Al and other nuclei 1

J

3

39–241

6–9

6

[3]

Spin system

2

J

J

Ref

27

Al–1H

27

Al– H

26

–

–

[3]

27

Al–11B

–

9

–

[3]

27

Al– C

7–191

1–15

–

[3, 112, 113]

27

Al–14N

22–45

–

–

[3]

27

Al–15N

44

–

–

[112]

27

Al–19F

18–20

–

–

[3]

27

Al–29Si

–

1–4

–

[114]

27

Al–31P

92–300

11–53

–

[3, 115, 116]

27

Al–35Cl

650

–

–

[3]

27

Al–81Br

750

–

–

[3]

2

13

q0

Figure 1

Si 3 q O SiOAlOSi O

O OAlO O q1 O OAlO 2 q O O Si OAlOSi O Si

85

80

Q0 O OSiO O

−70

Si q4 O SiOAlOSi O Si

75

70 65 60 55 d 27Al/ppm Si Q3 O Q2 SiOSiOSi O O SiOSiOSi O

Q1

50

45

Si Q4 O SiOSiOSi O Si

O OSiOSi O

−80

−90 −100 29 d Si/ppm

−110

Figure 2

AlR3

AlR4

Alkyls, silyls, & hydrides

AlR5

AlRn n ≥ 6 AlP4

AlP6

AlP5

P-donors

AlN4 AlN5

N-donors AlN6

AlS4

AlS6 AlX4 AlX5

S-donors

AlI4 Halides AlX6

AlO4

O-donors

AlO5 AlO6

300

250

200

150

100

50  27Al (ppm)

0

−50

−100

Figure 3

(a)

P1, F1

(b)

P2, F2 t1

t2

1 0 -1

1 0 -1

-p

-p

(d)

P2, F2 P3, F3 F4 t 1’

t 1”

P/2

t2

F3 t1

p

P1, F1

P2, F2

Acq

p

(c)

P1, F1

Acq

P1, F1

5QMAS

z-filter

3QMAS

p

p

1 0 -1

1 0 -1

-p

-p

z-filter

P/2

t2

Acq

P2, F2 t 1’

t + t 1”

t 1’

t

F3 P

P

t2

t 1”

Acq

t2

Figure 4

(a)

(90°x)sel

t = mTR

27Al

(90°±y)sel

(180°x)sel

t

90°x

180°x

t

31P

(b) 27Al

(90°)sel

(180°)sel

t = mTR

t

(180°)sel 31P

Acq

t

Acq

Figure 5

1.90

(Å)

= –0.0040 d + 2.08

r2 = 0.99

1.85 e -Al 13

Zunyite

1.80 Al(OH)4–

Al 30 g -Al 13

1.75 1.70 60

65

70

75

d (ppm)

80

85

Figure 6

Al13 Ke-J

Al Monomer

20 °C

Figure 7

60°

Figure 8

(a) 16 ppm

pH = 7.2

(b)

6.4

AlMo6(OH)6O183-

Al3+

Al(OH)3(S)

1.00

5.1

x Al

0.75

4.0

0.50 0.25

0 ppm

3.0

0.00 1

2

3

4

pH

1.6

1.2

x 512 40

30

20

10

0

-10

-20

d 27Al (ppm)

5

6

7

Figure 9

Figure 10

x16

(g) x8

(f) x4

(e) x4

(d) x2

(c) (b)

(a)

Figure 11

tm = 100 ms

q0

q1

q0

q2linear

tm = 10 ms

q1

q2cyclic

Figure 12

10 mm Vespel NMR tube

Teflon insert & stopper

Figure 13

(a) NH4AlPO4F

ii

Al2P2 SBU

ii

(b)

(c)

i

iii

i

i

30 °C 50 °C 100 °C

− 2H2O

− H2O

T

140 °C 175 °C

iii

ii 80

(d)

40

ii

210 °C 0 (ppm)

−40

−80

ii 80

40

0 (ppm)

AlV

−40

−80

AlV

(e)

AlVI

AlV

80

40 0 (ppm)

−40

iii

iv: Al2P2 SBU

80

40

0 −40 (ppm)

−80

Figure 14

(a) Hexameric SBU PO4 AlO6

AlO5

(b)

t (h)/180 °C 24 h 18 h

12 h 6h 0h 60

50

40

30 (ppm)

20

10

0

-10

70

60

50

40

30 (ppm)

20

10

0

Figure 15

Figure 16

Local structure in solid

(a)

(b)

Solid: XRD MIL-96 MIL-100 MIL-110

MIL-96

Species in solution

Liquid: 27Al NMR 7 ppm 4 ppm 0-1 ppm

7 ppm

MIL-100

Synthesis I

Solid Liquid −1 0

5

15

20

25 t (h)

4 ppm

Synthesis II

Solid Liquid

−1 0

5

15

20

25 t (h)

MIL-110

Figure 17

20

0

−20

−40 27Al

−60

−80

dimension (ppm)

−100

−120

Graphical Abstract

B0

54.74° From solution to materials

Highlights • The use of 27Al NMR spectroscopy is reviewed for the last 25 years (1990-2015). • NMR parameters of various classes of aluminum compounds are compiled. • Liquid-state and solid-state NMR complement each other. • Relevance of 27Al NMR is demonstrated in examples of typical aluminum chemistry.