MetalâOrganic Frameworks as Heterogeneous Catalysts in Hydrogen

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Metal-Organic Frameworks as heterogeneous catalysts in hydrogen production from lightweight inorganic hydrides Andrea Rossin, Giulia Tuci, Lapo Luconi, and Giuliano Giambastiani ACS Catal., Just Accepted Manuscript • Publication Date (Web): 19 Jun 2017 Downloaded from http://pubs.acs.org on June 19, 2017

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Metal-Organic Frameworks as heterogeneous catalysts in hydrogen production from lightweight inorganic hydrides Andrea Rossin,*,a Giulia Tuci,a Lapo Luconi,a Giuliano Giambastiani a,b a

Istituto di Chimica dei Composti Organometallici – Consiglio Nazionale delle Ricerche (ICCOM - CNR), Via Madonna del Piano 10, 50019, Sesto Fiorentino, Italy. b

Kazan Federal University, Kremlyovskaya Str. 18, 420008 Kazan, Russia

ABSTRACT: Ammonia borane (NH3·BH3, AB), hydrazine (NH2-NH2), lithium borohydride [Li(BH4)] and sodium alanate [Na(AlH4)] are popular chemical hydrogen storage inorganic solid materials featured by high gravimetric hydrogen contents (H wt.%) and remarkable stability at ambient conditions. Ultrapure H2 is formed from these compounds either via pyrolysis (i.e. a simple material heating) or via hydrolysis (chemical reaction with water). In both cases, a series of homogeneous and heterogeneous catalysts have been designed to assist the process. Among the latter, Metal-Organic Frameworks (MOFs, crystalline 3D porous lattices made of metallic nodes and organic polytopic linkers) have rapidly emerged as versatile candidates for this role. The nanoconfinement of lightweight hydrides in MOFs produces a “hydride@MOF” composite

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material. Hydride coordination to MOFs exposed metal sites or its reaction with functional groups on the organic linkers facilitates the thermal decomposition, lowering the hydrogen release temperature and increasing the hydrogen production rate. As for hydrolysis, MOFs are used as templates for the preparation of metal(0) nanoparticles (NPs) uniformly distributed in their inner cavities through a preliminary impregnation with a solution containing a metal salt followed by reduction. The “NPs@MOF” are the real active species that catalyze the reaction between the hydride and water, with concomitant H2 evolution. This perspective highlights the most representative literature examples of MOFs as heterogeneous catalysts (or catalyst supports) for H2 production from inorganic lightweight hydrides. Future trends in the field will also be discussed.

KEYWORDS: Metal-Organic Frameworks – Hydrogen – Ammonia borane – Hydrazine – Borohydrides – Alanates

Introduction One of the central global socio–economic challenges of the next decades is the sustainable supply of energy. In this respect, advances in hydrogen technology such as the generation of hydrogen from suitable starting materials, its storage and conversion to electricity are a central component. In view of growing fuel demand with very limited fossil fuel resources and environmental misbalance due to the over-emission of greenhouse gases (CO2), the quest for novel materials for hydrogen production from chemical reservoirs in the frame of the emerging “Hydrogen Economy” is constantly increasing. After its production, hydrogen (as an energy vector) must be distributed worldwide. Unfortunately, its handling and transportation for on-


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board automotive applications is difficult (due to its physical properties). To solve this problem, an intense research activity in the last 20 years has focused on finding new stable and non-toxic materials where hydrogen could be stored and easily released on demand. Lightweight hydrides containing metals from Groups 1 and 2 (Li, Na, Mg, Ca) and elements from Group 13/15 (B, Al, N) of the Periodic Table have attracted considerable interest in this regard, as they offer an appropriate balance of volumetric and gravimetric energy densities – both essential components of a practicable energy storage system. A number of reviews covering this topic have appeared during

the

last

ten

years,

highlighting

different

physico-chemical

aspects

and

applications.1,2,3,4,5,6,7,8,9,10,11,12,13 These compounds show high gravimetric hydrogen storage capacity and bear protic (N−H) and hydridic (B−H) hydrogens. When present together, their opposite polarity triggers weak intermolecular interactions called dihydrogen bonding (DHB): N−Hδ+···Hδ-−B. The presence of DHB facilitates hydrogen release via thermal decomposition (pyrolysis) under mild temperature conditions. An alternative approach to the generation of H2 is the direct reaction of lightweight hydrides with water (hydrolysis). In all cases, H2 produced by these compounds is of ultra-high purity (there are no carbonaceous contaminants). Ammonia borane (BH3·NH3, AB) is potentially an excellent carrier for hydrogen. AB contains up to 19.5 wt.% H; it is a non-flammable and stable crystalline solid. For AB, the ideal decomposition reaction is BH3·NH3 → “BN” + 3H2

(Eq. 1)

even if complete conversion to boron nitride (BN) is never reached in practice. The amount of H2 equivalents per AB molecule is thus ≤ 3. In the real system, complex poly(aminoboranes) featured by linear (BxNxH4x+2) / cyclic (BxNxH4x) structures, borazine (B3N3H6, the “inorganic


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benzene”) or polyborazylenes [BNHx (x < 2)] are found as reaction intermediates, very often as a solid mixture. Each of these species is related to the amount of H2 equivalents per AB equivalent produced during the catalytic process. In other words, the extent of AB dehydrogenation is the factor that regulates the chemical nature of the final by-product(s). The identification of these species is usually achieved through

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B NMR spectroscopy (Figure 1), either in solution (when

the compounds are soluble in the reaction solvent) or in the solid state (for insoluble species).

Figure 1. A pedagogical example of 11B NMR spectrum of AB thermal dehydrogenation by-products, with related assignments. Reprinted with permission from reference [24]. Copyright 2016 American Chemical Society.

Hydrazine (NH2-NH2) is a promising hydrogen carrier due to its relatively high hydrogen content and easy recharging properties.14,15 Hydrazine is a liquid that could be easily distributed through the existing infrastructures already designed for liquid hydrocarbons. Anhydrous hydrazine (12.5 wt.% H) decomposes following two alternative routes: NH2-NH2 → N2 (g) + 2 H2 (g) (complete decomposition)

(Eq. 2)


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3 NH2-NH2 → N2 (g) + 4 NH3 (g) (incomplete decomposition)

(Eq. 3)

From the viewpoint of hydrogen storage applications, Equation (3) should be avoided, in order to produce ammonia-free hydrogen. Literature studies have shown that the catalyst active surface plays a significant role to facilitate the reaction process selectively towards the two alternative decomposition reaction pathways expressed by Equations 2 and 3.16,17 Anhydrous hydrazine though is a dangerous liquid, being potentially explosive (especially when in contact with a metal catalyst). However, hydrous hydrazine (hydrazine monohydrate, NH2-NH2·H2O) is much safer and still contains a large amount of hydrogen (8.0 wt% H). The production of only nitrogen (which is not a greenhouse gas) besides hydrogen is one of the main advantages in the employ of hydrazine monohydrate.18,19 Borohydrides [M(BH4); M = Li, Na] are widely exploited as chemical hydrogen storage materials. Li(BH4) has a theoretical hydrogen capacity very close to that of AB (18.3 wt.% H), even if its practical application is limited by the high thermal decomposition temperature (about 380 °C) and its prohibitive re-hydrogenation conditions (pH2 = 350 bar and T = 650 °C). The thermal decomposition process is not always clear; many complex by-products form while H2 is released. According to the results of a computational study performed in 2005 by Kang and coworkers,20 the lowest energy path for hydrogen abstraction from LiBH4 is the following: LiBH4 → “LiBH” + 3/2 H2

(Eq. 4)

LiBH → “LiB” + ½ H2

(Eq. 5)


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The phase stability of LiBH4 and LiBH on the same Pnma orthorhombic structure is an important aspect in lithium borohydride dehydrogenation, and it favors the formation of this specific intermediate with respect to the hexagonal phase Li3BH6 (R3ത) and elemental boron. Aluminium hydrides (alanates) [M(AlH4); M = Li, Na] of light alkaline metals are another family of hydrogen storage materials extensively studied in the literature.21 Not all of them though are suitable for practical applications. LiAlH4 for example is kinetically stabilized (it has a slow H2 production kinetics). A much better candidate from this point of view is sodium alanate (NaAlH4); it is still featured by a high gravimetric hydrogen capacity (7.4 wt.% H) and by less energy-costly regeneration procedures. The decomposition of sodium alanate takes place in two steps (Equations 6 and 7) and involves formation of a stable intermediate (Na3AlH6). The dehydrogenation kinetics can be enhanced by addition of catalysts of assorted nature. NaAlH4 (aq) → 1/3 Na3AlH6 + 2/3 Al + H2

(Eq. 6)

Na3AlH6 → 3NaH + Al + 3/2 H2

(Eq. 7)

Alternatively, the direct reaction of water with lightweight hydrides (hydrolysis) also leads to (pure) hydrogen as one of the products. AB and NaBH4 hydrolysis for example can be expressed as follows:22,23 BH3·NH3 + 2 H2O → NH4BO2 + 3H2

(Eq. 8)

NaBH4 + 2 H2O → NaBO2 + 4H2

(Eq. 9)

A big effort has been made in the last 20 years to find suitable catalysts that lower the H2 release temperature (at least below 100 °C, or even ambient) for the pyrolysis process or improve the TON/TOF values (i.e. the reaction rate and H2 yield) in the hydrolysis reactions. A wide


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range of homogeneous24 and heterogeneous25 options is now available. Among the second class, Metal-Organic Frameworks (MOFs) have rapidly emerged as versatile and efficient heterogeneous catalysts for H2 production from lightweight hydrides. MOFs are highly porous, crystalline and self-organizing reticular structures composed of metal-based building units sharing polytopic organic linkers. Different assemblies between inorganic and organic components are conceivable, and examples of multivariate MOFs containing assorted metal nodes combined with the same linker or different spacers linking the same metal ions are present in the literature (Figure 2).26 They hold great potential both as catalyst supports and as catalysts themselves, because they can display many of the properties of an ideal heterogeneous catalyst, the most notable being crystallinity, active-site uniformity, high surface area and porosity.

Figure 2. Schematic picture of multivariate Metal-Organic Frameworks assembly, either from mixed metal nodes (a) or from mixed linkers (b).

Mesoporous MOFs, when compared to purely microporous materials, facilitate faster transport (diffusion) of molecular reactants and products, making these materials attractive also in the context of liquid-phase reactions (like hydrides hydrolysis). Two parallel approaches have


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been followed for lightweight hydrides dehydrogenation. The first one consists of a simple hydride loading into the MOF uniform pores through impregnation of a suitable solution (normally in ethers like THF), followed by solvent evaporation to get the final hydride@MOF composite (in Figure 3 the AB example is illustrated). Afterwards, the composite is heated and the H2 release temperature is monitored and compared with that of the pristine hydride to test the MOF catalytic effect in the pyrolysis.

Figure 3. Preparation of AB@MOFs composites.

The second one sees the MOF as a scaffold (support) for the deposition of metal nanoparticles (NPs) uniformly distributed in the regular pores of the MOF 3D structure.27,28 The NPs are prepared through a preliminary impregnation with a solution containing the metal salt (generally a simple metal halide), followed by solvent evaporation and successive reduction with a suitable reducing agent to form the M(0) NPs in situ (Figure 4). The as-prepared NPs@MOF composite is the final catalyst for hydrides hydrolysis at ambient temperature conditions.


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Figure 4. Preparation of M@MOFs catalysts.

In this Perspective, we will focus on the employ of both hydride@MOF and NPs@MOF composite materials as heterogeneous catalysts in the hydrides pyrolysis/hydrolysis for H2 production. The literature examples have been organized according to the hydride type. The activity and selectivity of each catalytic material will be presented as well, to get a wider and complete picture of the state-of-the-art in this field of investigation. Finally, the future research directions along this investigation path will be outlined and critically commented.

Ammonia borane dehydrogenation with MOFs: pyrolysis of AB@MOF composites Ammonia borane (AB) is one of the most popular inorganic hydrides exploited for H2 production to date. The main goals to be achieved with this material are the reduction of the first dehydrogenation temperature below T = 85 °C (as set by the U.S. Department of Energy – DOE), the increase of the hydrogen release rate and the suppression of detrimental volatile byproducts such as ammonia (NH3), diborane (B2H6) and borazine (B3N3H6). The elimination of


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byproducts in particular is a very important issue, since their traces (ppm level) poison the catalysts operating in PEM Fuel Cells where AB should be employed as a fuel. AB nanoconfinement in ordered porous solids like MOFs is a very smart approach to tackle the abovementioned problems, enhancing the hydrogen release kinetics and preventing byproducts formation. MOF-5, the pioneering founder of the MOFs dynasty made of [Zn4O]6+ metallic nodes and terephthalate (BDC2-) linkers,29 has been impregnated with a methanolic solution of AB and subsequently dried at room temperature (Figure 5).30,31 The final AB@MOF-5 composite has been thoroughly characterized with the usual solid state techniques. The BET surface areas drop down dramatically from 2170 m2/g (MOF-5) to only 7 m2/g (AB@MOF-5), confirming the complete MOF-5 pores filling with AB molecules.

Figure 5. AB@MOF-5 composite. Reproduced from ref. [30] with permission from the Royal Society of Chemistry.

Thermogravimetric analysis combined with mass spectrometry (TG-MS) has revealed that hydrogen production from AB@MOF-5 starts from T = 55 °C, a temperature considerably lower than that of pure AB (110 °C). Temperature-programmed desorption (TPD) experiments carried


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out at various heating rates have led to the experimental estimation of the H2 desorption activation energy (64.3 kJ mol-1), lower than that of bulk AB (184 kJ mol-1). This result confirms the improvement of the hydrogen generation kinetics through nanoconfinement in MOF-5. Borazine formation (m/z = 80 u) is totally suppressed at T = 55 °C, with evolution of ultra-pure hydrogen. Selective formation of poly(aminoboranes) occurs with this material while heating, as confirmed by

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B MAS-NMR spectra. This is reasonable, since the tubular channels present in

MOF-5 crystalline structure favor a (linear) polymerization process instead of cyclization to the more sterically encumbered borazine/polyborazylenes. The regular microstructure of the MOF is a key factor that ultimately influences the dehydrogenation byproduct structure. Consequently, the hydrogen “quality” produced through pyrolysis is intimately linked with the MOF scaffold morphology. During AB@MOF-5 pyrolysis, ammonia (m/z = 17 u) is still formed together with H2. In order to get its complete suppression, AB has been loaded into a MOF made of Y3+ ions combined with 1,3,5-benzenetricarboxylate (BTC3-) spacers denoted as JUC-32-Y.32 JUC-32-Y is featured by a regular microporous structure, high BET surface area, high thermal stability and (most importantly) open unsaturated metal sites formed upon coordinated water removal (preactivation at T = 300 °C for 4 h). AB was loaded into JUC-32-Y by infusion of methanolic 0.5 M solutions. The decomposition of AB in the AB@JUC-32-Y composite (Figure 6) starts from T = 50 °C, and no volatile products were found during the decomposition process.33 The absence of ammonia is justified as a consequence of the direct interaction of AB with the yttrium sites from its −NH3 terminus.


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Figure 6. AB@JUC-32-Y composite. Adapted with permission from reference [33]. Copyright 2010 American Chemical Society.

Some members of the “MIL” big MOF family (MIL = Matériaux Institute Lavoisier) have also been exploited within this context. MIL-101(Cr) is a chromium-based mesoporous MOF containing the BDC2- linker, featured by a high thermal stability (up to T = 275 °C), high surface area (> 2500 m2/g) and two hydrophilic zeotypic cavities of ca. 2.9 and 3.4 nm diameter size.34 In addition, the presence of many unsaturated chromium sites after pre-activation is an additional advantage for H2 production at low temperatures and at high rates. After impregnation of the activated MIL-101(Cr) with THF solutions and solvent evaporation at room temperature, AB fills 90% of the MOF pore volume, as inferred from the dramatic decrease of AB@MIL101(Cr) BET surface area (4.3 m2/g) with respect to the pristine form. From TG-MS analysis, the lowest hydrogen release temperature is 50 °C. Neither borazine nor diborane are detected during the pyrolysis. The activation energy for H2 desorption calculated from the experimental data equals 91.4 kJ mol-1. The interaction between AB and the MOF was analysed through UV-Vis and XPS spectroscopies. The visible absorption bands observed in the AB@MIL-101(Cr) spectrum come from Cr3+ and they are blue-shifted as compared with those of MIL-101(Cr). The blue shift is a proof of AB and its dehydrogenated forms coordination to the open metal sites in


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the MOF inorganic nodes (Figure 7). Further proof of evidence comes from B(1s) XPS spectroscopy, featured by a peak centred at 192.0 eV assigned to a B−O bond.35

Figure 7. Interaction of NH2=BH2 (the inorganic ethylene analogue) with the Cr3 node of MIL-101(Cr). Reprinted from reference [35]. Copyright 2011 Elsevier.

The system has been modified by Xu and co-workers in 2012 by embedding platinum NPs within MIL-101(Cr).36 The resulting Pt@MIL-101(Cr) hierarchical structure has been then impregnated with AB solutions in liquid ammonia, to obtain the AB@[Pt@MIL-101(Cr)] composite after solvent evaporation. The combination of nanoconfinement in the MOF mesopores and the nanoparticles catalytic effects lowered the first decomposition temperature to 88 °C. From the TG weight loss, it was inferred that 2 H2 equivalents per AB monomer are released reaching T = 180 °C, with significant suppression of the volatile by-products. An additional variation on the theme is that proposed the year after by Chan et al., where AB was loaded in amino-/acetamide-functionalized MIL-101(Cr), to study the effect of the linker functionalization on the hydrogen production performance.37 The –NH2 functionalized MOF showed a first H2 release temperature of 73 °C, while the sample bearing –NHCOCH3 groups on


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the terephthalate linkers produced H2 starting from T = 85 °C. Quantum mechanical calculations performed on the system show that a simple hydrogen bonding weak interaction is formed between the –NH3 terminus of AB and the lone pair on the amino group in MIL-101(Cr)-NH2. In the case of MIL-101(Cr)-NHCOCH3, the carbonyl group on the amide substituent is effective in AB activation through donation of one electron to the B atom. Flexible (“breathing”) MOFs have been used for AB nanoconfinement and dehydrogenation. Flexible MOFs can expand their lattice upon guest adsorption, adapting their pore opening to accommodate it. The breathing effect produces a dramatic cell volume modification without loss of crystallinity. MIL-53(Fe)38 is an iron(III)-based MOF composed of infinite inorganic chains made of [FeO4(OH)2] octahedra cross-linked by BDC2- organic spacers. The resulting open framework shows 1D diamondshaped channels with flexible behaviour. AB was loaded in MIL-53(Fe) following the usual impregnation procedure, and the homogeneous guest distribution within the MOF pores proved via powder X-ray diffraction (PXRD) and IR spectroscopy.39 AB@MIL-53(Fe) (Figure 8) can release 1.22 H2 equivalents per AB monomer at T = 80 °C with an instantaneous reaction, at odds with the long incubation period in pristine AB. The activation energy of H2 release is around 130 kJ mol-1. It is increasing with the AB loading, presumably because of the higher extent of AB-AB intermolecular interactions among the confined AB molecules.


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Figure 8. AB@MIL-53(Fe). Reproduced from ref. [39] with permission from The Royal Society of Chemistry.

One of the major drawbacks of MOFs as scaffolds to confine AB is the presence of a heavy (transition) metal as the constituent of the MOF inorganic nodes. This reduces the H2 gravimetric capacity of the AB@MOF system. Thus, it is highly desirable to have a lightweight MOF made of elements from Groups 1 and 2 of the Periodic Table with big pore sizes and high BET surface area. Mg-MOF-7440 belongs to this class. This magnesium-based MOF has a rigid framework made of 1D hexagonal channels with a diameter of approximately 12 Å running parallel to the organic 2,5-dioxo-terephtalate linker. The Mg2+ ions have a [MgO6] octahedral coordination environment, with five oxygen atoms coming from the linker and one oxygen atom coming from a coordinated water molecule. The material thermal activation creates open metal sites after water removal. The open sites may then interact with assorted small molecules, including AB. Impregnation techniques have led to the obtainment of the AB@Mg-MOF-74


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composite (Figure 9), showing a H2 release temperature < 70 °C, with no signs of volatile byproducts.41 The same group also considered the zinc analogue Zn-MOF-74 in 2012 for the same task.42 Again, a considerable reduction of the dehydrogenation temperature to T = 60 °C was recorded for the AB@Zn-MOF-74 system, with complete suppression of ammonia, borazine and diborane.

Figure 9. AB@Mg-MOF-74. Reproduced with permission from ref. [41]. Wiley-VCH GmbH & Co (2011).

The two MOFs of cubic (pcu) topology [Ni(4,4′-bipy)][HBTC] and [Ni(pyz)][Ni(CN)4] (4,4′-bipy = 4,4′-bipyridyl; pyz = pyrazine) were loaded with AB and the resulting composite tested in H2 production in 2010 by Li’s group.43 Since AB may also act as a mild reducing agent, the loading procedure causes the partial formation of Ni(0) NPs inside the MOF pores. These particles are the real active catalysts that enhance the kinetics and reduce the AB first dehydrogenation temperature to T = 80 °C for [Ni(4,4′-bipy)][HBTC] and to T = 102 °C for [Ni(pyz)][Ni(CN)4], respectively. The corresponding activation energies derived from the DSC experimental data are 131 and 160 kJ mol-1. The catalytic activity is ascribed to the NPs and not to the MOFs themselves, because the iron 3D framework Ni3[Fe(CN)6]2 (that does not undergo


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reduction of its metallic sites upon treatment with AB solutions) is catalytically inert under the same reaction conditions. A thullium MOF, Tm(BTC), was loaded with AB and tested as a pyrolysis catalyst in 2015 by Li and co-workers.44 Two different loading approaches were employed and compared: impregnation with a methanolic solution and solid-state physical milling. Tm(BTC) is a 3D crystalline polymer made of 6 x 6 Å 1D tubular channels and permanent porosity. Terminally coordinated aquo ligands on the Tm3+ ions can be easily removed upon thermal activation, leaving a reactive material with open metal sites.32 The impregnated sample showed the best catalytic performance, reducing the H2 release temperature to T = 60 °C, improving the reaction rate and preventing ammonia formation (that could not be avoided in the case of the ball-milled sample). The MOF porous structure is a key factor that reduces the pyrolysis temperature, since the “reference” AB@Tm2O3 (non-porous thullium oxide) sample was found to work at T = 80 °C. Direct interaction of AB with the exposed Tm3+ sites is invoked again to justify the absence of volatile byproducts. In the same year, the group of Wang loaded AB via ball-milling in the solid state within the 2D layered framework of [Ni(HBTC)(DMF)6] (DMF = N,N–dimethylformamide).45 This coordination polymer is featured by infinite extended sheets along the ab-plane. The sheets stack along the c-axis to give honeycomb hexagonal channels with an approximate distance of 7 Å between adjacent layers. The AB@[Ni(HBTC)(DMF)6] composite was analysed in terms of H2 production rate and yield. The nanoconfinement of AB in the polymer channels lowers the H2 release temperature to T = 70 °C and eliminates borazine / ammonia / diborane evolution.46


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Zeolite-imidazolate frameworks (ZIFs) is another class of MOFs that has been exploited for AB dehydrogenation. ZIFs are made of imidazole-containing linkers and assorted metal ions; they are featured by microporous structures with sodalite topology (as purely inorganic zeolites). In 2012, Burrell, Del Sesto and collaborators from Los Alamos (USA) have loaded AB into ZIF8 [Zn(mim)2, mim = 2-methylimidazolate]. This MOF is one of the most popular MOFs because it is chemically robust, thermally stable (up to 500 °C) and commercially available. It is featured by high surface area (≈1950 m2/g), large cavities of 11.6 Å diameter size and small pore apertures (3.4 Å).47 Different samples with increasing AB loadings from 6 to 90 wt.% were prepared via hand milling in the solid state. The TPD-MS profiles of the thermal dehydrogenation revealed that the first dehydrogenation event occurs at T = 85-88 °C for the low-loading samples (between 6 and 75 AB wt.%).48 Table 1 collects the relevant parameters of the catalysts described in this section, together with the related literature references. Table 1. Collective summary of the AB thermal dehydrogenation catalysts described in this Section.

First H2 release H2 wt.% produced temperature (°C)

MOF

Ref.

MOF-5

55

9.2 (@ T = 85 °C)

[30,31]

JUC-32-Y

50

8.0 (@ T = 85 °C)

[33]

MIL-101(Cr)

50

4.6 (@ T = 85 °C)

[35]

Pt@MIL-101(Cr)

88

6.5 (@ T = 180 °C)

[36]

MIL-101(Cr)-NH2

73

≈ 7.5 (@ T = 85°C)

[37]

MIL-101(Cr)-NHCOCH3

85

≈ 5.0 (@ T = 85°C)

[37]

MIL-53(Fe)

80

---

[39]

Mg-MOF-74

< 70

≈ 8.0 (@ T = 85°C)

[41]

Zn-MOF-74

60

≈ 9.0 (@ T = 85°C)

[42]

[Ni(bipy)][HBTC]

80

7.5 (@ T = 80°C)

[43]


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[Ni(pyz)][Ni(CN)4]

102

6.0 (@ T = 80°C)

[43]

Tm(BTC)

60

12.9 (@ T = 80°C)

[44]

[Ni(HBTC)(DMF)6]

70

≈ 6.0 (@ T = 90°C)

[46]

ZIF-8

85

---

[48]


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Ammonia borane dehydrogenation with MOFs: hydrolysis on M@MOF catalysts The reaction of AB with water (hydrolysis) is the alternative path chemists can follow to produce molecular hydrogen. The common approach used in this case is the preliminary preparation of metal(0) NPs embedded in the MOF pores, followed by treatment of the new M@MOF heterogeneous catalyst with an aqueous AB solution, generally at ambient temperature conditions. As reported in the previous paragraph, the MIL family has been extensively used as solid support for this purpose. MIL-101(Cr) in particular is a very popular choice for this application, because its giant pores facilitate a uniform and thorough diffusion of the NPs precursors through the MOF cavities. Ultra-small and uniformly dispersed NPs are eventually obtained. In 2013, the group of Xu and co-workers prepared AuNi alloy NPs supported on MIL-101(Cr).49 The advantage in the use of bimetallic NPs stems from the reduced amount of noble metal employed for their preparation, besides offering numerous opportunities for the modulation of their electronic structures and catalytic performance. They were prepared through impregnation of the MOF with aqueous solutions of HAuCl4 and NiCl2 metal precursors, followed by in situ reduction with NaBH4. The crystalline integrity of the framework is preserved after the impregnation/reduction treatment, as confirmed by PXRD measurements. TEM images confirmed the uniform distribution of the ultrafine particles within the MOF cavities. In the hydrolysis reaction, it is demonstrated that the AuNi@MIL-101(Cr) “bimetallic” catalyst is more active than its monometallic counterparts, exhibiting synergistic effects between gold and nickel. The most active species gives a turnover frequency (TOF) value of 66.2 molH2·(molAuNi)-1·min-1.


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One year later, the same group prepared bimetallic AuCo NPs loaded on MIL-101(Cr) using the same approach and starting from HAuCl4 and CoCl2 solutions.50 The appreciable decrease of the BET surface area when passing from MIL-101(Cr) to AuCo@MIL-101(Cr) (1930 m2/g) indicates that NPs are highly dispersed in the MOF framework. Again, in AB hydrolysis at ambient temperature the bimetallic catalyst shows a better performance than its monometallic analogues, with a final TOF value of 23.5 molH2·(molAuCo)-1·min-1. Mixed-metal core-shell Pd@Co NPs have also been loaded on MIL-101(Cr) in 2015, starting from Pd(NO3)2 and CoCl2 solutions.51 The obtainment of core-shell NPs was achieved through successive impregnation/reduction cycles based on the different reduction potentials of the two metals. The resulting [Pd@Co]@MIL-101(Cr) system (Figure 10) has a superior catalytic activity and excellent recyclability in AB hydrolysis if compared to its monometallic counterparts and also to the PdCo@MIL-101(Cr) catalyst (where the two metals are present as a simple alloy and not as core-shell nanostructures), proving again the existence of a synergistic effect between the two metals. The best TOF value achieved is 51 molH2·(molPdCo)-1·min-1. The activation energy for H2 production estimated from the Arrhenius plots of the reaction rate constants at different temperatures is calculated to be 22 kJ mol-1.


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Figure 10. Preparation of [Pd@Co]@MIL-101(Cr). Reproduced with permission from ref. [51]. Wiley-VCH GmbH & Co (2015).

The same authors prepared non-noble CuCo bimetallic NPs with different molar ratios loaded on MIL-101(Cr) starting from Cu(NO3)2 and CoCl2 salts.52 The highest TOF value achieved in AB hydrolysis is 19.6 molH2·(molCuCo)-1·min-1. The CuCo@MIL-101(Cr) heterogenous catalyst also showed high durability and recyclability. More recently, a variation on the theme was made through incorporation of ultrafine NiRu bimetallic NPs within the same MOF, using RuCl3 and NiCl2 precursors.53 The synthetic approach is a combination of a doublesolvent (n-hexane/water) impregnation [where the precursor compounds can easily diffuse through the pores of MIL-101(Cr) by capillary force] followed by an overwhelming reduction with a large excess of reducing agent NaBH4. This method avoids formation of NPs on the external MOF surface. Different NiRu@MIL-101(Cr) samples with variable Ru/Ni compositions were then tested in AB hydrolysis at room temperature. The highest catalytic activity was found for the Ni:Ru molar ratio of 70:30, with a TOF value of 272.7 molH2·(molNiRu)-1·min-1. Matsuyama et al. immobilized ultrafine and highly dispersed platinum NPs inside the pores of MIL-101(Cr) starting from H2PtCl6 solutions and adopting a supercritical CO2 (scCO2)assisted impregnation and drying approach.54 ScCO2 drying improves the MOF pore stability and, at the same time, it increases the available surface area. Indeed, a comparison of the H2 release rates from AB hydrolysis on Pt@MIL-101(Cr) samples prepared through the conventional / scCO2 methods reveals that the latter are faster under the same experimental conditions.


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The group of Yamashita and co-workers deposited non-noble Cu / Co / Ni NPs on MIL101(Cr) samples functionalized with amino groups on the organic linker [i.e. 2-aminoterephalic acid: NH2-MIL-101(Cr), that is a photo-sensitive material], through the usual impregnation method starting from nitrate salts.55 The target the authors had in mind was the preparation of plasmonic photocatalysts that could operate under visible light irradiation (λ > 420 nm), converting solar energy into chemical energy at ambient temperature. The M@NH2-MIL101(Cr) catalysts were very efficient in AB hydrolysis at ambient temperature and under visible light irradiation; the highest TOF value recorded is that found for Ni@NH2-MIL-101(Cr) [54.0 molH2·(molNi)-1·min-1]. All samples exhibited remarkable durability and recyclability. The MOF scaffold not only acts as a NPs support but also as a semiconductor for visible light harvesting. The same group also reported about deposition of bimetallic gold/palladium NPs on the same MOF support.56 Gold has attracted remarkable attention in this context as a fascinating photo-responsive material that can strongly harvest a wide range of visible light frequencies. Within bimetallic AuPd NPs, gold enhances the activity of Pd-catalysed chemical reactions under the assistance of visible light by transferring its photo-excited electrons to palladium. In AB hydrolysis, the AuPd@NH2-MIL-101(Cr) system showed the highest dehydrogenation rate when compared with its monometallic counterparts. An additional variation on the same theme is the MOF doping with heteroelements within its inorganic nodes. The partial substitution of suitable metal ions in the nodes can greatly improve the performance in redox reactions, due to the efficient charge separation created by oxo-bridges in the metal cluster. The same authors prepared cerium-doped NH2-MIL-101(Cr) and deposited palladium NPs on this support. The resulting Pd@NH2-MIL-


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101(Ce/Cr) system was then tested in AB hydrolysis under visible light irradiation at ambient temperature, with a TOF value of 39.3 molH2·(molPd)-1·min-1.57 A different member of the MIL family was finally selected by the same group as a scaffold for the deposition of Pd NPs: NH2-MIL-125(Ti). This MOF has minimal formula [Ti8O8(OH)4(NH2-BDC)6] and it is featured by large pores of both tetrahedral (6 Å size) and octahedral (12.5 Å size) shape and good water stability.58 Several Pd@NH2-MIL-125(Ti) heterogeneous catalysts prepared with different procedures (but with the same metal content of 0.25 Pd wt.%) were tested under the same conditions; the best TOF value recorded is 52.5 molH2·(molPd)-1·min-1 in the hydrolysis of AB at ambient temperature.59 Lu and co-workers prepared bimetallic RuCo NPs supported on MIL-96(Al) starting from RuCl3 and CoCl2 salts. MIL-96(Al) is an aluminium-containing MOF of minimal formula Al12O(OH)18(H2O)3(Al2(OH)4)(BTC)6. It is a 3D framework containing isolated trinuclear µ3oxo-bridged aluminum clusters and infinite chains of AlO4(OH)2 and AlO2(OH)4 octahedra forming a honeycomb lattice based on 18-membered rings in the (a,b) plane. The two types of aluminum groups are connected to each other through the trimesate linkers (BTC3-). Two types of cages are of interest for catalytic purposes, with estimated pore volumes of 417 and 635 Å3.60 The RuCo@MIL-96(Al) system is featured by a TON value of 320.7 molH2·(molRu)-1·min-1 in the hydrolysis of AB at room temperature, with an activation energy of 36.0 kJ mol-1. As observed in other similar cases, the monometallic counterparts Co@MIL-96(Al) and Ru@MIL-96(Al) were less active, confirming the synergistic effect of the two metals within the alloy. The H2 generation rate vs. Ru:Co molar ratio plot showed a “volcano”-type trend, the best performance being recorded for the 1:1 sample. This indicates that neither Ru nor Co has enough activity for catalysing AB hydrolysis at complete conversion.61


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The UiO family (UiO = University of Oslo) is made of zirconium octahedral nodes [Zr6 = Zr6O4(OH)4(RCOO)12] linked together by dicarboxylate linkers of different length and bearing various functional groups. They all possess cubic crystal symmetry, with tetrahedral and octahedral cages in a 2:1 ratio with dimensions that depend on the length of the linker. They are featured by a remarkable thermal stability and resistance to water and aggressive chemical reagents like concentrated strong acids or bases. The simplest example is UiO-66, where terephthalate spacers (BDC2-) are used to connect the metal nodes.62 The bimetallic AgPd system was loaded by Wang and co-workers on the amino-functionalized UiO-66 (UiO-66-NH2, containing the 2-aminoterephtalate linker), starting from AgNO3 and PdCl2 solutions. Different samples with variable Ag:Pd molar ratios were prepared and characterized. The most active for AB hydrolysis at ambient temperature was found to be AgPd4@UiO-66-NH2, with a measured TOF value of 90 molH2·(molAgPd)-1·min-1. An activation energy value of 51.77 kJ mol-1 for H2 production was estimated from the Arrhenius plot lnk vs. 1/T, after performing reactions at different temperatures between 20 and 35 °C.63 In 2012, nickel NPs have been loaded into ZIF-8 using a volatile nickelocene precursor (Cp2Ni; Cp = η5-C5H5-) diffused through the MOF cavities via chemical vapor deposition (CVD).64 The Ni(II) compound was then reduced to Ni(0) NPs with H2 at T = 300 °C for 3 h. Different samples of Ni@ZIF-8 were prepared, with varying metal loading (measured through ICP analysis). The most active in AB hydrolysis was found to be the sample with a Ni loading of 19.0 wt.%, giving a TOF value of 14.2 molH2·(molNi)-1·min-1 and completing the reaction in 13 minutes only. The very high TOF values found confirm that the ZIF-8 framework effectively immobilize the nanoparticles within its pores, preventing undesired agglomeration or sintering of the active catalyst.


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The same scaffold was loaded with bimetallic RhNi nanoparticles in 2015 by Luo and coworkers.65 Highly dispersed NPs with an average diameter of 1.1 ± 0.2 nm were anchored on ZIF-8 via liquid impregnation of RhCl3 / NiCl2 methanolic solutions followed by reduction at 0 °C with NaBH4. Samples featured by different Rh:Ni ratios were prepared. The highest TOF [58.8 molH2·(molRhNi)-1·min-1] obtained in AB hydrolysis at ambient temperature was recorded for the Rh15Ni85@ZIF-8 sample. Table 2 collects the relevant parameters of the catalysts described in this section, together with the related literature references. Table 2. Collective summary of the AB hydrolysis catalysts described in this Section.

M@MOF

TOF value for AB hydrolysis [molH2·(molM)-1·min-1]

Ref.

AuNi@MIL-101(Cr)

66.2a

[49]

AuCo@MIL-101(Cr)

a

[50]

[Pd@Co]@MIL-101(Cr) CuCo@MIL-101(Cr) NiRu@MIL-101(Cr)

51

a

19.6

[51] a

272.7

a

Pt@MIL-101(Cr)

---

Ni@NH2-MIL-101(Cr)

54.0b

AuPd@NH2-MIL-101(Cr) Pd@NH2-MIL-101(Ce/Cr) Pd@NH2-MIL-125(Ti) RuCo@MIL-96(Al) AgPd4@UiO-66-NH2 Ni@ZIF-8 Rh15Ni85@ZIF-8 a

23.5

[52] [53] [54] [55]

b,c

[56]

39.3

d

[57]

52.5

d

[59]

18.8

320.7

a

[61]

90.0

a

[63]

14.2

a

[64]

e

[65]

58.8

Calculated at complete AB conversion; b calculated at maximum reaction rate; c estimated on the

basis of reported data; d calculated at 83-84 % AB conversion; e calculated at 50 % AB conversion.


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Hydrazine dehydrogenation with MOFs While for ammonia borane a significant number of literature examples can be found, the case studies of H2 production from hydrazine using MOFs as catalysts or catalyst supports are scarce. Other types of supports (inorganic oxides, zeolites, carbon nanomaterials)66 have been used instead in the past to load metal NPs (of assorted composition and with both non-noble and noble transition metals). Many studies have been conducted with unsupported NPs (i.e. a simple NPs suspension).67,68 As already mentioned in the Introduction, in this context it is of fundamental importance to design heterogeneous catalysts that selectively produce H2 over NH3 (see Equations 2 and 3 above). Luo and co-workers prepared NiPt bimetallic NPs anchored to MIL-101(Cr) through impregnation with aqueous NiCl2 / K2PtCl6 solutions followed by reduction with NaBH4 at T = 0 °C.69 The as-synthesized NiPt@MIL-101(Cr) systems with variable Ni:Pt ratios were tested in NH2-NH2·H2O decomposition at T = 50 °C in basic solutions (NaOH 0.5M. Figure 11). The pure-nickel-containing MOF [Ni100@MIL-101(Cr)] is inactive, while progressive addition of growing amounts of platinum increased both activity and H2 selectivity. Ni88Pt12@MIL-101(Cr) is the most active and selective (100 % H2) sample, with a TOF value of 65.2 molH2·(molNiPt)-1·h1

at ambient temperature and an activation energy for H2 evolution of 51.29 kJ mol-1 (derived

from experiments carried out at different temperatures in the 25 – 60 °C range). Too high Pt amounts decrease the selectivity: the pure-platinum-containing MOF [Pt100@MIL-101(Cr)] does not release hydrogen at all. This confirms the positive synergistic effect of the simultaneous presence of both metals on the catalyst performance.


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Figure 11. Aqueous hydrazine dehydrogenation by NiPt@MIL101(Cr). Reprinted with permission from reference [69]. Copyright 2014 American Chemical Society. .

Bimetallic NPs made of nickel and another noble metal (Pt, Rh) were prepared and loaded on ZIF-8. The group of Xu in Osaka (Japan) prepared and characterized the NiPt@ZIF-8 system. Subsequently, it was exploited for hydrazine selective dehydrogenation in aqueous alkaline solution at T = 50 °C.70 Hydrous hydrazine could be completely converted to H2 and N2 in only 26 minutes by Ni80Pt20@ZIF-8, with a TOF of 90 molH2·(molNiPt)-1·h-1. Interestingly, the purely monometallic catalysts are inactive under the same conditions. Alloying nickel with a small amount of platinum tunes the catalyst surface electronic structure, improving its performance. Luo and collaborators in Wuhan (China) prepared NiRh@ZIF-8, using the same approach described above for the NiPt systems. The highest activity was found for the Ni66Rh34@ZIF-8 sample, with a TOF value reaching 140 molH2·(molNiRh)-1·h-1 and 100 % hydrogen selectivity at T = 50 °C.71 The activation energy for H2 evolution is 58.1 kJ mol-1 under these conditions.


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Borohydrides and alanates dehydrogenation with MOFs As stated in the case of hydrazine, the literature examples of H2 production from borohydrides or alanates using MOFs as catalysts or catalyst supports are rare to date. Other types of heterogeneous catalysts employed for this task are listed in a collective review published by Wang and co-workers in 2013 (but their description is out of the scope of this Perspective).72 The reader is redirected there for further information. Allendorf and co-workers published the first example of a MOF as catalyst in 2009.73 They loaded NaAlH4 in the copper-based MOF HKUST-1 (HKUST = Hong-Kong University of Science and Technology), also known as “copper-BTC”.74 This MOF has minimal formula [Cu3(BTC)2] and it was chosen because its pore openings are much smaller than the interior dimensions. 4 pores per unit cell are present, with 9 x 9 Å openings and 11 x 16 Å interior dimensions. This should limit the alanate particles mobility, providing ultra-finely dispersed material into the MOF cavities. HKUST-1 is also thermally stable up to 250 °C. The MOF was soaked into a THF solution of sodium alanate; the infiltrated material was then heated at T = 110 °C to evaporate the solvent, yielding a sample of NaAlH4@HKUST-1 with 4.0 wt.% hydride (from elemental analysis). A series of solid-state techniques (FTIR, PXRD, TEM/EELS, MAS-NMR, porosimetry) has been exploited to characterize the composite material and prove the uniform particle distribution within the MOF pores. The H2 desorption behavior of NaAlH4@HKUST-1 has been finally examined via TGMS. It is very different from that of bulk NaAlH4: while neat sodium alanate evolves H2 starting from T = 250 °C, its confinement into HKUST-1 lowers the H2 production temperature at only 70 °C. The nanoscale effects result from a combination of reduced particle size and local chemical environment confining the clusters. In fact, HKUST-1 possesses open Cu(II) sites that


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are formed after its thermal activation. The direct interaction of the hydride with the copper ions is thought to have a strong (beneficial) effect on the H2 release temperature decrease. Yu and co-workers used the same MOF scaffold to load LiBH4 in 2011.75 The preactivated MOF was suspended in a THF solution of lithium borohydride to achieve uniform dispersion and contact. After ultrasonic treatment for 10 minutes, the suspension was kept under vacuum for 3 h at room temperature, to get the target composite LiBH4@HKUST-1. The first dehydrogenation event occurs at T = 60 °C, much lower than that of pristine LiBH4 (380 °C). The direct interaction of the borohydride with the MOF copper open sites is invoked to explain this catalytic effect (Figure 12). No diborane by-product is formed during the borohydride dehydrogenation, although no evidence of any B-containing byproduct and of a clear decomposition mechanism are provided; the authors claim that the high catalytic activity of HKUST-1 is due to the interaction between LiBH4 and the Cu−O bonds in the MOF.

Figure 12. Proposed interaction of LiBH4 with the exposed Cu(II) sites in HKUST-1. Reproduced from ref. [75] with permission from The Royal Society of Chemistry.


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More recently, Allendorf and co-workers exploited the same approach to prepare the nano-NaAlH4@Mg-MOF-74 composite.76 Given the lower MOF molecular weight, high hydride loadings could be achieved (up to 21 wt.%) using a melt infiltration technique. The MOF is intimately ground with the hydride, and the solid mixture sealed in a stainless steel autoclave. Heating at T = 195 °C under H2 pressure (to avoid hydride decomposition) induces the hydride melting and consequent infiltration into the MOF pores. The confinement of NaAlH4 in the pores eliminates the stable intermediate Na3AlH6 phase during the thermal decomposition and the reaction proceeds directly to NaH, Al and H2, in agreement with theory. The onset for H2 desorption is around T = 50 °C, much lower than that of bulk sodium alanate (150 °C). The activation energy for H2 evolution from nanoconfined NaAlH4 is 57.4 kJ mol-1.

Summary and outlook Lightweight inorganic hydrides are interesting chemical hydrogen storage materials. The economic and scientific relevance of hydrogen as a green and “zero-carbon” energy source is at the core of the technology platforms on energy. Many countries worldwide have launched a series of actions to support the development of a new energy strategy. Under such perspective, new and better-performing heterogeneous catalysts for lightweight hydrides dehydrogenation are currently being designed and tested in many chemical laboratories of the World. MOFs are porous materials that can be prepared ad hoc, with a specific application in mind. The extreme versatility of combination of their building blocks (both organic and inorganic) makes them attractive as catalysts.


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In order to raise the wt.% of the loaded substrate, stable MOFs featured by light constituting metallic elements (Li, Be, Mg) and by high porosity / inner surface area could be exploited as NPs scaffolds or active catalysts themselves for hydrides dehydrogenation after impregnation/loading. A good candidate belonging to this group is the beryllium analogue of MOF-5, called MOF-5(Be), prepared by the group of Mertens in 2010.77 It is featured by high thermal stability (up to T = 500 °C), a BET surface area of 3289 m2/g and it has minimal formula [Be4O(BDC)3]. The same tetrahedral [M4O]6+ metal node as that found in ordinary MOF-5 is present. The resulting 3D cubic framework is isoreticular to that of MOF-5 (“IRMOF”). Another suitable option is the lithium-based MOF [Li2(H2BTB)2] (denoted as CPM-46; H3BTB = benzene-1,3,5-tribenzoic acid). It is made of 1D channels of hexagonal symmetry running along the [001] Miller direction and featured by Li2 dimers acting as a 6-connected node linked by tritopic BTB ligands. It shows a BET surface area of 592 m2/g.78 As for magnesium-containing MOFs, good candidates could be [Mg3(NDC)3] (NDC2- = 2,6-naphtalenedicarboxylate; Langmuir surface area = 520 m2/g; stable to T = 500 °C) prepared by Kaskel and collaborators in 200679 or [Mg3(BHTC)2] (BHTC3- = biphenyl-3,4′,5-tricarboxylate; BET surface area = 1650 m2/g), published by Matzger et al. in 2010.80 The light metal in the inorganic nodes improves the gravimetric hydride content of the resulting hydride@MOF composite, and consequently the amount of H2 produced per MOF formula unit. Another future development on this research field that may be promising in terms of efficiency and H2 production selectivity is the incorporation of active functionalities into thermally and chemically stable MOFs, such as the zirconium-based UiO family. Recent work by Fahra and Hupp in Northwestern University (Chicago – USA)81 has demonstrated that all MOF components (linkers, nodes, pores) can be modified to prepare new catalytic materials for


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target reactions. Following this approach, single-site catalytic units in the form of complex metal hydrides82,83,84 could be embedded into UiO MOFs through anchoring to the metallic [Zr6] node via solvent-assisted ligand incorporation (SALI), producing more efficient and more selective “supported” catalysts for AB, hydrazine, borohydrides and alanates dehydrogenation (Figure 13). Alternatively, the same organometallic fragments could be attached to the MOF linker when suitable anchoring points are present (a process better defined as “ligand postsynthetic metalation”, Figure 14). Complex metal hydrides are the ideal catalytic species for H2 production from those substrates bearing protic H atoms (hydrazine, AB). They facilitate hydrogen evolution through engagement into Hδ+···Hδ- dihydrogen bonding. In summary, chemists can follow many different and complementary experimental approaches to prepare better MOF-based catalysts for hydrogen production from inorganic lightweight hydrides. This is a growing field of investigation still at its early beginning, and many interesting results are expected to come in the near future.

Figure 13. SALI approach for the anchoring of complex metal hydrides to the metallic nodes of UiO-like MOFs.


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Figure 14. Postsynthetic ligand metalation to attach complex metal hydrides to the linkers of UiO-like MOFs.

Corresponding Author *[email protected] ORCID ID: 0000-0002-1283-2803 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Acknowledgments Our research on the reviewed topics (thiazole-based MOFs synthesis and applications in catalysis; AB dehydrogenation by complex metal hydrides) is supported by grants from the CNR-RFBR Italy-Russian Federation bilateral project of the Italian National Research Council and from the Italian MIUR through the PRIN 2015 Project SMARTNESS (2015K7FZLH) “Solar driven chemistry: new materials for photo- and electro-catalysis”.


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References (1) Johnson, H. C.; Hooper, T. N.; Weller, A. S. Top. Organomet. Chem. 2015, 49, 153-220. (2) Moury, R.; Demirci, U. B. Energies 2015, 8, 3118-3141. (3) Zhao, Q.; Li, J.; Hamilton, E. J. M.; Chen, X. J. Organomet. Chem. 2015, 798, 24-29. (4) Jespen, L. H.; Ley, M. B.; Lee, Y.-S.; Cho, Y. W.; Dornheim, M.; Jensen, J. O.; Filinchuk, Y.; Jørgensen, J. E.; Besenbacher, F.; Jensen, T. R. Mater. Today 2014, 17, 129-135. (5) Chen, X.; Zhao, J.-C.; Shore, S. G. Acc. Chem. Res. 2013, 46, 2666-2675. (6) Stubbs, N. E.; Robertson, A. P. M.; Leitao, E. M.; Manners, I. J. Organomet. Chem. 2013, 730, 84-89. (7) Lan, R.; Irvine, J. T. S.; Tao, S. Int. J. Hydrogen Energy 2012, 37, 1482-1494. (8) Smythe, N. C.; Gordon, J. C. Eur. J. Inorg. Chem. 2010, 509-521. (9) Staubitz, A.; Robertson, A. P. M.; Manners, I. Chem. Rev. 2010, 110, 4079-4124. (10) Umegaki, T.; Yan, J.-M.; Zhang, X.-B.; Shioyama, H.; Kuriyama, N.; Xu, Q. Int. J. Hydrogen Energy 2009, 34, 2303-2311. (11) Peng, B.; Chen, J. Energy Environ. Sci. 2008, 1, 479-483. (12) Marder, T. Angew. Chem. Int. Ed. 2007, 46, 8116-8118. (13) Stephens, F. H.; Pons, V.; Baker, R. T. Dalton Trans. 2007, 2613-2626. (14) Yadav, M.; Xu, Q. Energy Environ. Sci. 2012, 5, 9698-9725.


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Table of Contents and Synopsis

Metal-Organic Frameworks (MOFs) have rapidly emerged as versatile heterogeneous catalysts for H2 production from pyrolysis or hydrolysis of lightweight inorganic hydrides: ammonia borane, hydrazine, borohydrides and alanates. This Perspective collects the literature examples in the field and provides a critical outlook for future design of better-performing systems.


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Figure 1. A pedagogical example of 11B NMR spectrum of AB thermal dehydrogenation by-products, with related assignments. Reprinted with permission from reference [24]. Copyright 2016 American Chemical Society. 79x44mm (300 x 300 DPI)


ACS Catalysis

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Figure 2. Schematic picture of multivariate Metal-Organic Frameworks assembly, either from mixed metal nodes (a) or from mixed linkers (b). 50x41mm (300 x 300 DPI)


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Figure 3. Preparation of AB@MOFs composites. 456x106mm (96 x 96 DPI)


ACS Catalysis

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Figure 4. Preparation of M@MOFs catalysts. 460x224mm (96 x 96 DPI)


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ACS Catalysis

Figure 5. AB@MOF-5 composite. Reproduced from ref. [30] with permission from the Royal Society of Chemistry. 342x135mm (72 x 72 DPI)


ACS Catalysis

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Figure 6. AB@JUC-32-Y composite. Adapted with permission from reference [33]. Copyright 2010 American Chemical Society. 33x32mm (300 x 300 DPI)


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ACS Catalysis

Figure 7. Interaction of NH2=BH2 (the inorganic ethylene analogue) with the Cr3 node of MIL-101(Cr). Reprinted from reference [35]. Copyright 2011 Elsevier. 146x141mm (96 x 96 DPI)


ACS Catalysis

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Figure 8. AB@MIL-53(Fe). Reproduced from ref. [39] with permission from The Royal Society of Chemistry. 138x193mm (72 x 72 DPI)


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ACS Catalysis

Figure 9. AB@Mg-MOF-74. Reproduced with permission from ref. [41]. Wiley-VCH GmbH & Co (2011). 71x48mm (300 x 300 DPI)


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Figure 10. Preparation of [Pd@Co]@MIL-101(Cr). Reproduced with permission from ref. [51]. Wiley-VCH GmbH & Co (2015). 238x132mm (300 x 300 DPI)


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ACS Catalysis

Figure 11. Aqueous hydrazine dehydrogenation by NiPt@MIL-101(Cr). Reprinted with permission from reference [69]. Copyright 2014 American Chemical Society. 60x47mm (300 x 300 DPI)


ACS Catalysis

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Figure 12. Proposed interaction of LiBH4 with the exposed Cu(II) sites in HKUST-1. Reproduced from ref. [75] with permission from The Royal Society of Chemistry. 222x200mm (300 x 300 DPI)


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ACS Catalysis

Figure 13. SALI approach for the anchoring of complex metal hydrides to the metallic nodes of UiO-like MOFs. 456x229mm (96 x 96 DPI)


ACS Catalysis

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Figure 14. Postsynthetic ligand metalation to attach complex metal hydrides to the linkers of UiO-like MOFs. 420x168mm (96 x 96 DPI)


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TOC Graphic 101x100mm (96 x 96 DPI)


MetalâOrganic Frameworks as Heterogeneous Catalysts in Hydrogen

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