Metal–Organic Frameworks as Heterogeneous Catalysts in Hydrogen

Perspective pubs.acs.org/acscatalysis

Metal−Organic Frameworks as Heterogeneous Catalysts in Hydrogen Production from Lightweight Inorganic Hydrides Andrea Rossin,*,† Giulia Tuci,† Lapo Luconi,† and Giuliano Giambastiani†,‡ †

Istituto di Chimica dei Composti Organometallici, Consiglio Nazionale delle Ricerche (ICCOM-CNR), Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy ‡ Kazan Federal University, Kremlyovskaya Str. 18, 420008 Kazan, Russia ABSTRACT: Ammonia−borane (NH3·BH3, AB), hydrazine (NH2NH2), lithium borohydride (Li(BH4)), and sodium alanate (Na(AlH4)) are popular chemical hydrogen storage inorganic solid materials featuring high gravimetric hydrogen contents (H wt %) and remarkable stability under 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 material. Hydride coordination to MOF 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. 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

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physicochemical aspects and applications.1−13 These compounds show high gravimetric hydrogen storage capacity and bear protic (N−H) and hydridic (B−H) hydrogens. When these hydrogens are 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 ultrahigh 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 nonflammable and stable crystalline solid. For AB, the ideal decomposition reaction is

INTRODUCTION One of the central global socioeconomic challenges of the next decades is a sustainable supply of energy. In this respect, advances in hydrogen technology such as the generation of hydrogen from suitable starting materials and its storage and conversion to electricity are central components. In view of growing fuel demand with very limited fossil fuel resources and environmental misbalance due to the overemission of greenhouse gases (CO2), the quest for novel materials for hydrogen production from chemical reservoirs in the framework 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-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 nontoxic 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 10 years, highlighting different © 2017 American Chemical Society

BH3· NH3 → “BN” + 3H 2

(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 polyReceived: May 7, 2017 Revised: June 8, 2017 Published: June 19, 2017 5035

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and co-workers,20 the lowest energy path for hydrogen abstraction from LiBH4 is

(aminoboranes) featured by linear (B x N x H 4x+2 )/cyclic (BxNxH4x) structures, borazine (B3N3H6, the “inorganic 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 byproduct(s). The identification of these species is usually achieved through 11B NMR spectroscopy (Figure 1), either in solution (when the compounds are soluble in the reaction solvent) or in the solid state (for insoluble species).

LiBH → “LiB” + 1/2H 2

(5)

NaAlH4(aq) → 1/3Na3AlH6 + 2/3Al + H 2

(6)

Na3AlH6 → 3NaH + Al + 3/2H 2

(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 as22,23

Hydrazine (NH2NH2) 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:

BH3· NH3 + 2H 2O → NH4BO2 + 3H 2

(8)

NaBH4 + 2H 2O → NaBO2 + 4H 2

(9)

A great deal of 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 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. Mesoporous MOFs, in comparison 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 (such as hydride hydrolysis). Two parallel approaches have been followed for lightweight hydride dehydrogenation. The first consists of a simple hydride loading into the MOF uniform pores through

complete decomposition (2)

3NH 2NH 2 → N2(g) + 4NH3(g)

(4)

The phase stability of LiBH4 and LiBH in the same Pnma orthorhombic structure is an important aspect of lithium borohydride dehydrogenation, and it favors the formation of this specific intermediate with respect to the hexagonal phase Li3BH6 (R3̅) and elemental boron. Aluminum 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 still features a high gravimetric hydrogen capacity (7.4 wt % H) and less energy costly regeneration procedures. The decomposition of sodium alanate takes place in two steps (eqs 6 and 7) and involves formation of a stable intermediate (Na3AlH6). The dehydrogenation kinetics can be enhanced by addition of catalysts of assorted nature.

Figure 1. Pedagogical example of the 11B NMR spectrum of AB thermal dehydrogenation byproducts, with related assignments. Reprinted with permission from ref 24. Copyright 2016 American Chemical Society.

NH 2NH 2 → N2(g) + 2H 2(g)

LiBH4 → “LiBH” + 3/2H 2

incomplete decomposition (3)

From the viewpoint of hydrogen storage applications, eq 3 should be avoided, in order to produce ammonia-free hydrogen. Literature studies have shown that the catalyst active surface plays a significant role in facilitating the reaction process selectively toward the two alternative decomposition reaction pathways expressed by eqs 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 hydrate, NH2NH2·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) in addition to hydrogen is one of the main advantages in the use of hydrazine hydrate.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 rehydrogenation conditions (pH2 = 350 bar and T = 650 °C). The thermal decomposition process is not always clear; many complex byproducts form while H2 is released. According to the results of a computational study performed in 2005 by Kang 5036

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hydrogen release rate, and the suppression of detrimental volatile byproducts such as ammonia (NH3), diborane (B2H6), and borazine (B3N3H6). The elimination of 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 such as MOFs is a very smart approach to tackle the aforementioned 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 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. 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). Temperatureprogrammed desorption (TPD) experiments carried out at various heating rates have led to the experimental estimation of the H2 desorption activation energy (64.3 kJ mol−1), which is 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) is totally suppressed at T = 55 °C, with evolution of ultrapure hydrogen. Selective formation of poly(aminoboranes) occurs with this material on heating, as confirmed by 11B MAS NMR spectra. This is reasonable, since the tubular channels present in the 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) 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

Figure 2. Schematic picture of multivariate metal−organic framework assembly, either from mixed metal nodes (a) or from mixed linkers (b).

impregnation of a suitable solution (normally in ethers such as THF), followed by solvent evaporation to get the final hydride@MOF composite (in Figure 3 the AB example is illustrated). Afterward, 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. The second approach 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 hydride hydrolysis under ambienttemperature conditions. In this Perspective, we will focus on the use of both hydride@MOF and NPs@MOF composite materials as heterogeneous catalysts in hydride 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.

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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 in the

Figure 3. Preparation of AB@MOF composites. 5037

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Figure 4. Preparation of M@MOF catalysts.

activation energy for H2 desorption calculated from the experimental data equals 91.4 kJ mol−1. The interaction between AB and the MOF was analyzed through UV−vis and XPS spectroscopy. The visible absorption bands observed in the AB@MIL-101(Cr) spectrum come from Cr3+, and they are blue-shifted in comparison with those of MIL-101(Cr). The blue shift is a proof of coordination of AB and its dehydrogenated forms to the open metal sites in the MOF inorganic nodes (Figure 7). Further proof comes from B(1s) XPS spectroscopy, featuring a peak centered at 192.0 eV assigned to a B−O bond.35

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

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

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

as a consequence of the direct interaction of AB with the yttrium sites from its −NH3 terminus. 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, featuring 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.34 In addition, the presence of many unsaturated chromium sites after preactivation 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 in AB@MIL-101(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 is detected during the pyrolysis. The

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 nanoparticle 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 on reaching T = 180 °C, with significant suppression of the volatile byproducts. 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 the terephthalate linkers produced H2 starting from T = 85 °C. Quantum 5038

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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 composite (Figure 9), showing a H2 release temperature of