Hydrogen Release from Ammonia Alane-Based Materials: Formation

Article pubs.acs.org/JPCC

Hydrogen Release from Ammonia Alane-Based Materials: Formation of Cyclotrialazane and Alazine Vinh Son Nguyen,† D. Majumdar,‡ Jerzy Leszczynski,‡ and Minh Tho Nguyen*,† †

Department of Chemistry, University of Leuven, B-3001 Leuven, Belgium Interdisciplinary Center for Nanotoxicity, Department of Chemistry, Jackson State University, Jackson, Mississippi 39217, United States

‡

S Supporting Information *

ABSTRACT: In previous papers (Nguyen et al. J. Phys. Chem. C 2008, 112, 5662−5667 and J. Phys. Chem. C 2009, 113, 18914− 18926), formation of H2 molecules from ammonia alane monomer (AAl) and dimers (AAl)2 was shown to be facilitated by the addition of one or more alane or ammonia molecules that can play the role of efficient bifunctional catalyst. Ammonia alane emerges as a good starting compound for building up materials for chemical hydrogen storage (CHS). Further exploration of the products based on the H2 release from ammonia alane were carried out using coupledcluster theory computations together with the aug-cc-pVTZ basis set (based on MP2/aug-cc-pVDZ optimized geometries). Our ab initio MO calculations for the first time led to the identification of cyclotrialazane [(H2AlNH2)3], alazine [(HAlNH)3], and its oligomer [H2Al(HNAlH)2NH2] that are produced along the multistep dehydrogenation processes from the reactions of ammonia alane and AlH3NH2AlH2NH3. The formation of alazine (homologue of borazine) as the final product in our H2 elimination reactions is an important feature because of its long-debated existence. The present reaction path analyses show that the formation of this compound is an important phenomenon for explaining the entire dehydrogenation process.

1. INTRODUCTION Ammonia has long been used as a convenient source of H2, and related investigations have appeared for the last 40 years. When the concept of a “hydrogen economy” started to be widely debated in the early 1970s, ammonia was envisioned as a perfect storage medium for molecular hydrogen produced by ocean thermal energy conversion. Ammonia has several desirable characteristics that make it attractive as a medium to store hydrogen. First, it is nonflammable and nonexplosive. Second, it can be liquefied under mild conditions, and this is less complex than reforming hydrocarbon fuels. Third, ammonia has a large weight fraction of hydrogen (H2 constitutes 17.65% of the mass of NH3). More importantly, ammonia is one of the materials that can be cheaply produced, efficiently transported, and directly transformed to yield H2 and nonpolluting byproducts.1−5 In this context, a class of compounds called “ammonia-based complexes” (ABC) is now considered as a solution for chemical hydrogen storage (CHS) materials. The ABC compounds containing only B or Al, N, and H atoms6−14 meet the requirements of high gravimetric material capacities for on-board hydrogen storage. Moreover, the lightweight boron and aluminum are also capable of attaching multiple hydrogens to them. Finally, the most critical point is that the B−H/Al−H and N−H bonds tend to be hydridic and protic, respectively. These properties normally facilitate hydrogen release15−21 to make such B/Al−N−H compounds effective CHS materials. © 2015 American Chemical Society

The main approach for making efficient CHS materials based on ammonia is to find hydride partners, which can form dative bonds with it to form stable adducts. The resulting adduct can then release H2 through hydridic and protic dihydrogen bonds. A rather simple route is to consider the light metal hydrides, which are similar to boranes. Among B/Al−N−H compounds, the light aluminum hydride alane (AlH3) is an attractive material for hydrogen storage. α-Alane is known to be kinetically stable in air for long periods of time.22−24 In this regard, the combination of ammonia with alane is an attractive avenue to be explored. Ammonia alane (AAl, NH3AlH3), which is isovalent with ammonia borane (AB), is kinetically stable and has in fact been considered as a promising material for CHS.25−34 It is to be noted that the lone pair of nitrogen and empty orbital of aluminum also play a role in reinforcing their net atomic charges. With the advantage of the resulting large difference in net atomic charges between both nitrogen (negatively charged) and aluminum (positively charged) atoms, ammonia alane features a capacity to give both hydrides and protons and thereby to facilitate H2 release, through H-transfer relays, by reacting with other compounds. The chemical formula of alane shows that it contains more than 10% of H2 (in theoretical weight percent), a Received: November 21, 2014 Revised: February 10, 2015 Published: February 11, 2015 4524

DOI: 10.1021/jp511668z J. Phys. Chem. C 2015, 119, 4524−4539

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The Journal of Physical Chemistry C

be an important characteristic of the resulting compounds during the dehydrogenation mechanism and constitutes a crucial step in creating novel CHS materials.

density of 148 g H2/L, and a decomposition temperature of ∼100 °C. It is more than double the density of liquid H2. Another reason of the general interest in this highly energetic compound is that it is abundantly available and primarily used for solid rocket propellants and explosives.35,36 Recently, researchers at Savannah River National Laboratory have found an electrochemical reversible route to synthesize aluminum hydride which not only provides a boost to hydrogen storage but also has other applications in synthetic chemistry.37 In another study, Zhao et al. reported that an aluminoborane AlB4H11 compound has been shown to be a good candidate for hydrogen storage materials.38 This synthetic route closes a reversible cycle for regeneration of the material and bypasses expensive thermodynamic costs which have so far precluded AlH3 from being considered as a H2 storage material.37 The heats of formation and reaction thermodynamic parameters of the molecular AAl and the corresponding ionic salt [AlH4]−[NH4]+ have been calculated in gas phase as well as in solid state using high-accuracy ab initio electronic structure theory. These results showed that these compounds release H2 through near-thermoneutral processes.25,26 Our previous calculations for a series of chemical reactions of AAl with several simple catalysts pointed out that it is a promising CHS material for which AlH3 was found to be a very effective catalyst. It was further demonstrated that, as a bifunctional catalyst, alane (AlH3) tends to accelerate H2 production from either the compounds C2H6, BH3NH3, BH3PH3 or AlH3NH3 and AlH3PH3.27,29,34,39−42 H2-elimination from ammonia-alane monomer and -dimer creates a number of intermediates as well as byproducts,29,33,39 which turn out to be a potential series of materials for CHS. Many of the works cited above pointed out that AAl not only is a good CHS compound but also plays a role as a key intermediate structure (monomer and dimer) in the growth mechanism of aluminum−nitrogen compounds. The inorganic butane analogue NH3BH2NH2BH3 (DDAB, a dehydrodimer of ammonia borane) is an important intermediate during the reaction of NH2BH2 with AB.43 DDAB was first synthesized from the reaction of amino-diborane (NH2B2H5) with ammonia, and the crystal structure analysis showed that it exits in a gauche conformation.43−47 DDAB is a candidate for CHS materials as it plays an important role in H2 generation during the formation of oligomeric species.34,43−47 Similarly, NH3AlH2NH2AlH3 (Gau, the alane analog of DDAB which exits in gauche form) is the most stable intermediate in the dehydrogenation of dialane (Al2H6, DAl) in the presence of two ammonia (DAl + 2A), or of ammonia alane dimers (AAl + AAl).34 Cyclotriborazane (CTB) and borazine (BZ) are the most interesting products in the dehydrogenation processes from ammonia borane reactions, followed by a “cyclic” route. They are the so-called “inorganic cyclohexane” and “inorganic benzene” structures, respectively.48−50 Although both CTB and BZ have long been investigated using both theoretical and experimental techniques,51−62 their structural analogs cyclotrialazane (CTA) and alazine (AZ) (occurring as end products during similar dehydrogenation processes) have not been explored in detail.63−65 Several theoretical investigations have pointed out the structural characteristics of AZ,66,67 formation of this trimer in CVD reactions,68 and its aromatic character.69 In view of the lack of information on CTA and AZ, the present work addresses their formation mechanism via dehydrogenation processes from the reaction of Gau and AAl using state-of-the-art quantum chemical techniques. A further objective is to explore different dehydrogenation channels involving the reactions of ammonia alane with AlH3NH2AlH2NH3. The dihydrogen bonding has been found to

2. METHODOLOGIES All the calculations were carried out using the Gaussian 0370 and Molpro 200671 suites of programs using the augmented correlation consistent basis sets72,73 (aug-cc-pVxZ, up to quadruple (Q) zeta-type) of the atoms. For the sake of brevity, these basis sets will be abbreviated as aVxZ (with x = D, T, and Q) throughout the manuscript. For all structures, geometries and vibrational frequencies were computed using the second-order perturbation theory (MP2)74 with the aVDZ basis set. The MP2/aVDZ frequency results were scaled by 0.97 to evaluate the zero-point energies (as described in our previous studies). Our previous studies33,34 further pointed out that geometrical parameters obtained at the MP2/aVDZ and MP2/aVTZ levels are quite close to each other; hence, the MP2/aVDZ data are used here for further analysis. Intrinsic reaction coordinate (IRC)75 profiles were constructed through density functional theory (DFT) using the hybrid B3LYP functional76,77 and the 6-311++G(d,p) basis set of atoms to ascertain the structures of the relevant transition-state structures (TSs). IRC calculations were carried out with the step size of 0.1 amu1/2 Bohr. The DFT/ B3LYP method was selected for IRC calculations because of the obvious advantage in terms of computing times. Relative energies were then calculated from single-point electronic energies at the CCSD(T)78−80 level using the optimized structures from MP2/aVDZ calculations using aVxZ basis sets. When possible, the final critical energies were obtained by extrapolating CCSD(T)/aVnZ energies to the complete basis set (CBS) limit by using the expression81 E(x) = A CBS + B exp[− (x − 1)] + C exp[− (x − 1)2 ] (1)

with x = 2, 3, and 4 for the aVxZ (x = D, T, and Q) basis sets. Relative energies based on CCSD(T)/aVTZ total energies differ by at most ±1.0 kcal/mol from our best estimates obtained at the CCSD(T)/CBS-aVxZ level. This agrees well with the results previously found for the smaller BH3NH3 and AlH3NH3 systems presented in our previous studies.14,29,33,34,39

3. RESULTS AND DISCUSSION 3.1. Dehydrogenation Mechanisms of Ammonia Alane and NH3AlH2NH2AlH3. The present section is organized in the following way. We first investigate the reaction of ammonia alane (AAl) and NH3AlH2NH2AlH3 (Gau). In this reaction, several H2 molecules are formed, and each intermediate structure can connect different TSs for H2-loss. To describe the dehydrogenation processes from AAl + Gau, we explore each H2-loss according to the number (n) of released H2 (n = 0 to 5). The following convention is adapted to abbreviate the equilibrium and transition structures. Each equilibrium structure is denoted by the first capital letter of the compound, namely G, followed by a number indicating the number of H2 released (n = 0 → 5), and a lower case number for different stable isomers. As per convention, each transition structure is denoted by G-TSxy in which G stands for the starting minima, followed by the letters TS for a transition structure, and x and y for the positions of the Al and N atoms, respectively, from which the H2 molecule is formed. 3.1.1. Formation of Complexes (n = 0). The shapes and selected geometrical parameters of the relevant equilibrium 4525


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Scheme 1. Shapes and Selected MP2/aVDZ Geometrical Parametersa of the Most Stable Complexes G01, G02, G03, G04, and G05 and Their Corresponding Lower Derivatives DADAl, IonP-ala, dim1-ala, dim2-ala, and dala-ben Formed from (NH3AlH3)2

a

Bond lengths are given in angstroms, and bond angles are in degrees.

structures are displayed in Scheme 1. The condensation of AAl and the gauche (coiled) structure of Gau yields five complexes denoted as G01, G02, G03, G04, and G05. They are lower than the sum of energies of the reactants AAl + Gau by 8−13 kcal/mol. The most important structural characteristic of these complexes is the presence of multiple hydrogen bonds, and their strengths are responsible for their relative stability order. The two most stable complexes G01 and G02 can be imagined as compounds having the forms of zwitterionic species DADAl and IonP-ala, respectively (cf. Scheme 1). The other three complexes G03, G04, and G05 can also be seen as structures similar to the dimeric forms of dim1-ala, dim2-ala, and dala-ben complexes, respectively (Scheme 1).29 They are considered as the lower derivatives of the respective G0n (n = 1−5) complexes as their formation involves the substitution of H atoms in the NH3, AlH3, NH2AlH3, AlH2NH3, and NH3 along with NH2AlH3 moieties of the respective zwitterions DADAl, IonP-ala, the dimers dim1-ala and dim2-ala, and the complex dala-ben. The similar and almost identical forms and shapes of G0n complexes with respect to their lower derivatives indicate, in accordance with our previous studies,28,34 that the formation of those complexes is energetically favorable. It has been shown that the zwitterions, dimers, and complexes (the lower derivatives) are capable of releasing H2 molecules.34 The relative stabilities of the G0n complexes arise from two factors which include the relative strengths of the hydrogen bonds and conformational energies of the individual components in the zwitterionic and dimeric forms. It is obvious from the structures (from hydrogen bond distances in Scheme 1) that hydrogen bonds in G0n are getting relatively weaker in going from G01 to G05. The energies

Table 1. Relative Energies of the Reactants, Complexes, Transition States, and Products of the Formation Complexes and the First H2-Loss Processes relative energy (including ZPE corrections) (kcal/mol) species reactants AAl + Gau complexes G01 G02 G03 G04 G05 TSs G01-TS23 G01-TS21 G01-TS12 G02-TS23 G02-TS13 G02-TS31 products G11 + H2 G12 + H2 G13 + H2 C01 + H2 C02 + H2

MP2/aVDZ

CCSD(T)/aVDZ

CCSD(T)/aVTZ

0.0

0.0

0.0

−14.2 −13.7 −10.9 −10.4 −8.1

−15.4 −14.8 −12.1 −11.6 −9.5

−13.1 −12.7 −10.2 −9.8 −8.0

10.2 13.0 26.3 10.3 13.5 14.2

10.6 13.6 27.5 10.7 14.2 14.9

11.6 13.8 27.9 11.2 14.5 15.5

−30.8 −24.2 −24.1 −27.3 −27.3

−30.0 −23.4 −23.2 −26.6 −26.7

−31.1 −24.8 −24.1 −26.5 −26.5

of the conformers of the individual components in the less stable complexes are also relatively higher with respect to their respective lowest-energy structures in G01 (cf. Table 1). 4526


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Figure 1. Schematic energy profiles illustrating six different reaction pathways for H2-release from two lowest complexes, G01 and G02. Relative energies in kilocalories per mole are from CCSD(T)/aug-cc-pVTZ + ZPE calculations.

G05 is a higher derivative of dala-ben and the least stable among the G0n complexes (5.1 kcal/mol above G01, Table 1). The lower stability is a consequence of weaker hydrogen bonding interactions here (Scheme 1) as well as zwitterionic contribution in hydrogen bonding. The results thus show that the zwitterionic forms tend to dominate in the relative stability order of these complexes. This is in line with the experimental results for ammonia borane dimer in the solid state.82,83 Moreover, in the homologous ammonia borane dimeric system,29 hydrogen eliminations are more favorable from zwitterionic compounds than from neutral dimeric forms. 3.1.2. The First H2-Loss (n = 1). We now explore the dehydrogenation pathways for the release of the first H2 from G01 and G02 complexes. Schematic energy profiles illustrating six different reaction pathways for H2-release starting from G01 and G02 are displayed in Figure 1. Twelve TSs can be derived from G01 and G02 within barrier heights from 10 to 38 kcal/mol. They represent two kinds of TSs related to the formation of H2 molecule through Al−H−H−N dihydrogen interactions and 1,2-H2 eliminations. They differ much from each other by the size of the corresponding energy barriers. Transition structures for H2 elimination are described by the position of H atoms during H-transfer, namely, head-to-tail and head-to-center. Geometrical parameters of these TSs and the resulting products

These are due to more eclipsing of the bonded hydrogens, which results in eclipsed structures of the components, thus generating van der Waals repulsion. Table 1 contains the energetic features of the complexes considered at various levels of theory. The relative energies are comparable with each other. Unless otherwise mentioned, the energetics at only the CCSD(T)/aVTZ + ZPE level will be used hereafter for the discussion. G01 is located at −13.1 kcal/mol below the reactant Gau + AAl energies, and the complexation energy of G02 is only 0.4 kcal/mol higher than that of G01 (Table 1). Both G01 and G02 have also the same kind of Al−H−H−N dihydrogen bonds in zwitterionic interactions (Scheme 1). The shortest distances of 1.66 and 1.65 Å in dihydrogen bonds are the head-to-tail interactions, and thereby they will facilitate the H2 release in the following step. G03 and G04 are the higher derivatives of dim1-ala and dim2-ala (cf. Scheme 1) with five dihydrogen bonds in the head-to-tail and head-to-center interactions. They are relatively less stable than G01 and G02 complexes (2.9 and 3.3 kcal/mol above G01, Table 1) as the dihydrogen bonds (1.727 to 2.857 Å) show much weaker interactions (longer distances) with respect to those of G01 and G02 (Scheme 1), and the zwitterionic character in such hydrogen bond formation is absent in these higher-energy complexes. 4527


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Figure 2. Selected MP2/aug-cc-pVDZ geometry parameters of six lowest transition structures and products related to H2-elimination from G01 and G02. Bond distances are in angstroms, and bond angles are in degrees.

processes via G01-TS23 or G02-TS23 lead to the products C01 + H2 and C02 + H2. They are both located at ∼26.5 kcal/mol below the separated reactants AAl + Gau. C01 and C02 compounds are not only the products of the H2 elimination processes via G01-TS23 and G02-TS23 but also two complexes of a condensation between Cyc (cyclic (NH2AlH2)2) and AAl. Although C01 (+ H2) and C02 (+ H2) are both thermodynamically and kinetically viable candidates as products, the reactions of Cyc with AAl and their dehydrogenations are quite complicated

are shown in Figure 2. Energies of the TSs and the products are all listed in Table 1. The TSs for 1,2-H2 eliminations are shown in Figure S-1 of the Supporting Information. Two H2-releasing TSs are located from either G01 or G02. They both have a head-to-tail form as discussed before. The lowest-lying TSs G01-TS23 and G02-TS23 (Figures 1 and 2) are formed through hydrogen interactions of AA1 with those of the Alhead and Ntail atoms of the Gau structure. Formation of H2 in these TSs is from the Gau moiety. The dehydrogenation 4528


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The Journal of Physical Chemistry C and need careful exploration. Therefore, in this paper the dehydrogenation processes from C01 and C02 will not be discussed further. G01-TS21 and G02-TS13 are 13.8 and 14.5 kcal/mol above the separated reactants AAl + Gau, respectively. H2 is formed from G01-TS21 by combination of H atoms from N2 of AAl and Al1 of Gau (Figure 1). A H2 is formed in the same way from G02-TS13 but from a reverse combination, i.e., H atom originating from Al1 of AAl and N3 of Gau moieties. The energy difference between these two TSs is very small (∼0.7 kcal/mol), and the IRC paths for both TSs show that they produce identical products (G11 + H2, cf. Figure 1). The formation of these end products could be considered as exothermic from energy consideration with respect to both G01 and separated reactants AAl + Gau. G11 has three dihydrogen bonds, and two of them are originated through interactions of the hydrogen atoms of NH3+ and the Al3 and Al2 hydrogens (Figure 2). The transition structures G01-TS12 and G02-TS31 are higher in energy than those of G02-TS13, and their respective end products G13 + H2 and G12 + H2 are also in an exothermic position (Figure 1). H2 formed from these TSs takes place through interactions of two head-to-center H atoms from AAl and Gau moieties. The higher barrier height of G01-TS12 could be explained from the charges on two H atoms of the dihydrogen bonds involved. In fact, the HAl1 and HN2 atoms in the Al-1(δ+)−H(δ+)−H(δ+)−N-2(δ−) dihydrogen bond of G01-TS12 carry the charges of +0.4 and +0.1 e, respectively (from Mulliken population analysis). On the contrary, the two atoms HAl3 and HN1 in the Al-3(δ+)−H(δ−)−H(δ+)−N-1(δ−) framework of G02-TS31 are susceptible to react with each other because of the opposite charges of −0.4 and +0.5 e, respectively, and obviously the resulting barrier is lower because of electrostatic stabilization. The charge difference (CD) between both hydrogen atoms in the Al−H−H−N dihydrogen bonds is also an approximate measure of relative energy barriers of the H2 release. The smaller the CD, the larger the barrier. This observation is consistent with an observation in our previous studies.84−86 All of the TSs displayed in Figure 2 show that dihyrogen bonds are consistently involved in H2 release, and the migration of one H atom from a NH3-head to the electron-deficient AlH3-tail invariably leads to H2 elimination. We will see throughout this work that this particular type of dihydrogen interaction plays a primary role in stabilizing the transition structures for hydrogen release. 3.1.3. Cyclotrialazane Formation from the Second H2-Loss. All possible pathways for dehydrogenation from G11, G12, and G13 are shown in Figure 3. Geometrical parameters of relevant TSs and resulting products are displayed in Figure 4a,b. Energies of reactants, TSs, and products computed at different levels are shown in Table 2. Again, relative positions of the stationary points remain unchanged with respect to the methods employed. Three starting materials G11, G12, and G13 are capable of hydrogen release through multiple TSs. The two most stable structures, G11 and G12, are very similar to the linear and branched six-membered ring structures previously reported by Sneddon and co-workers.87 The energies of the products show that all H2 release reactions are exothermic in nature, except for G22 + 2H2 formation from G13 + H2, which is endothermic. G11-TS21 is the lowestlying TS among the three TSs, namely, G11-TS21, G11-TS31, and G11-TS32, predicted for H2 release from G11. It has a six-membered ring structure due to a very short dihydrogen (N−H−H−Al) interaction (0.804 Å, Figure 4a). This means that

Table 2. Relative Energies of the Reactants, Complexes, Transition States, and Products of the Second H2-Loss Processes relative energy (including ZPE corrections) (kcal/mol) species reactants G11 G12 G13 TSs G11-TS21 G11-TS31 G11-TS32 G12-TS13 G12-TS12 G13-TS21 G13-TS23 G13-TS13 products G21t G22 G23 G24 G25 G26 G27

MP2/aVDZ

CCSD(T)/aVDZ

CCSD(T)/aVTZ

0.0 0.0 0.0

0.0 0.0 0.0

0.0 0.0 0.0

20.9 23.0 29.0 14.9 31.9 16.0 19.2 30.3

22.4 24.5 31.0 16.3 33.9 17.7 20.8 32.1

21.1 23.1 28.9 15.0 32.2 15.1 18.6 29.2

−17.6 −10.5 −16.2 −8.2 −14.0 −5.1 5.4

−15.1 −7.6 −13.6 −5.7 −11.2 −2.2 8.8

−18.8 −11.1 −17.1 −9.2 −14.6 −5.9 3.2

H2 is almost released via this TS. The H2 release from this TS leads to stable six-membered cyclic product G21 (Figures 3 and 4b). It is an analog of cyclotriborazane60 and exits as a twist boat conformer G21t (Figure 4b). According to our MP2/aVDZ data, the other possible chair conformer of G21 (G21c, Figure S-2 of the Supporting Information) is ∼0.5 kcal/mol higher in energy than G21t, and this observation is consistent with previous studies on similar systems.60 The structures of the two upper-lying TSs equally show that H2 is ready to be eliminated through very short dihydrogen bonds. In the case of G11-TS31 (having an imaginary frequency 582i cm−1), the corresponding H−H distance in N1−H−H−Al3 is 0.89 Å, whereas it is 0.82 Å in the Al2−H−H−N3 moiety of the highest-lying TS G11-TS32 (Figure 4a). Once the H2 is formed, the NH2AlH3 group of G11-TS31 rotates by ∼180° to form an Al−H−Al hydrogen bond to generate the final product G22 (Figures 3 and 4b). On the other hand, the departure of H2 occurs in the same plane as the AlN···AlN skeleton (similar to the transition structure for H2 loss from [AlH3NH2AlH2NH3] reported in ref 34) and then generates the final product G24. The product G22 has a four-membered cyclic form of (AlH2NH2)2 and is higher in energy than G21 (with a six-membered ring structure). G24 is similar to G22 with a four-membered ring structure. The main difference is that the former does not enjoy a stabilization effect due to the absence of any hydrogen bond; hence, it is higher in energy. G12 produces H2 molecules from two different TSs, G12-TS13 (having an imaginary frequency of 217i cm−1) and G12-TS12 (Figure 3). The barrier height of the former is much lower than that of the latter. The higher barrier height associated with G12-TS12 is due to the presence of two hydrogen atoms in close proximity on Al1, and formation of the resulting three membered ring generates tremendous strain to put it at an energy level higher than that of G12-TS13. The reactions are however exothermic, and G23 has a lower energy than G26. As in the previous cases, the TSs are almost in a position to lose H2 because 4529


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Figure 3. Potential energy surfaces (PESs) of H2-release from G11, G12, and G13. Relative energies in kilocalories per mole are from CCSD(T)/aug-ccpVTZ + ZPE calculations.

short dihydrogen bond (Figure 4a). The product G22 coming from G13-TS21 has the same structure and relative stability as that of the product from G11-TS31. Except for the orientation of the H−H moiety in the dihydrogen bond, the basic structures of the two TSs are similar to each other. This thus leads to an identical final product. The exothermic product G22 is more stable than G25 (formed from G13-TS23) because of stabilization through several dihydrogen bonds (even though both have similar strain from four-membered rings). The product G27, coming from the endothermic path through G13-TS13, is the least stable one because of the presence of two fused four-membered rings in its structure (Figure 4b). Thus, although G22 is not favored in the H2 elimination from G11, it is both kinetically and thermodynamically favored through H2-loss from G13. 3.1.4. Continuous Reactions Followed by the Third H2-Loss. The products of the second H2-loss processes, namely, G21t, G22, G23, G24, G25, G26, and G27, are capable of further hydrogen release, and we explore further in this section the reaction pathways for such dehydrogenation reactions. It should be mentioned that both structures G23 and G24 were found to be analogous to the branched aminoborane cyclic oligomer, B-(cyclodiborazanyl) amine borane structure (BCDB), proposed earlier by Baker and co-workers.88,89 However, G23 is devoid of any viable hydrogen generation capability; therefore, it is considered as one of the end products.

of the presence of very short dihydrogen bonds (H−H distances are 0.83 and 0.81 Å in G12-TS13 and G12-TS12, respectively; Figure 4a). The product G23 is similar to G26, but they are different from each other by the position of AlH3 group. The higher energy of G26 is due to the strain imparted by the presence of a four-membered ring and weak hydrogen bonding (Figure 4b). Thus, formation of G23 + 2H2 is both kinetically and thermodynamically viable. There are three possible channels for G13 to generate H2, involving three different transition states, namely, G13-TS21, G13-TS23, and G13-TS13. The TS G13-TS21 gives rise to the lowest barrier height, whereas G13-TS13 leads to the highest barrier (Figure 3). Energy barriers from such TSs are basically due to their stability ordering as individual species through various interactions and strain. G13-TS21 and G13-TS23 are stabilized through dihydrogen bond interactions as found from their geometries given in Figure 4a. G13-TS21 could have a higher relative stability due to a larger number of dihydrogen interactions. G13-TS13 is characterized by an even higher barrier, as it has lesser stability due to the presence of a four-membered ring structure (Figure 4a), which imparts strain in the system. The reactions through G13-TS21 and G13-TS23 are exothermic, whereas it is endothermic when the reaction proceeds through G13-TS13. Again, as in the two previous cases, H2 elimination reactions from G11 and G12 involve transition states having one 4530


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Figure 4. Selected MP2/aug-cc-pVDZ geometry parameters of (a) eight lowest transition structures and (b) seven products related to H2-eliminations from G11, G12, and G13. Bond distances are in angstroms, and bond angles are in degrees.

shapes, but the end product after H2 release turns out to be the same, namely the G31. Formation of G31 is slightly endothermic during the dehydrogenation of G21, whereas for the rest of the cases it is exothermic. The geometric features of the TSs G21-TS23, G22-TS13, G24-TS23, G25-TS21, and G26-TS23 (Figures 5a and 6a) show that the H2-generation is almost completed in these TSs because of very short dihydrogen bonds (having very short H−H distance). Another similarity of these dehydrogenation processes is that they can all be seen as 1−4 hydrogen elimination. G26-TS23 is associated with the lowest energy barrier, and the relative barrier heights of the other transition states can be explained as above by considering stabilization through dihydrogen bond interactions and a

The associated potential energy profiles for H2 release for the rest of the starting compounds are illustrated in Figure 5a,b. Selected geometrical parameters for the TSs and products of these processes are displayed in Figure 6a,b. Related energetics at different levels of theory are presented in Table 3. Again, a similar energy ordering can be found for all levels considered. Figure 5a,b indicates that G21t, G24, G25, and G26 can actually produce H2 through two different pathways, whereas for G22 and G27 only one channel is open. The schematic potential energy profiles in Figure 5a reveal an interesting feature. The TSs for hydrogen release from the starting compounds G21t, G22, G24, G25, and G26 could be different with different barrier heights and 4531


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Figure 5. Potential energy surfaces of H2-release from G2n (n = 1 → 7). Relative energies in kilocalories per mole are from CCSD(T)/aug-cc-pVTZ + ZPE calculations.

elimination and as the result of a bond formation between N3 and Al2. The resulting structure is stabilized through nonplanarity to avoid ring strain and is marginally higher in energy than G21t structure (by 1.3 kcal/mol).

strain imposed due to the presence of smaller ring structures (Figure 6a). G31 is an analog of the Gau structure in which the HAl and HN atoms are replaced by NH2 and AlH2 groups, respectively. The product is generated from 1−4 hydrogen 4532


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Figure 6. Selected MP2/aug-cc-pVDZ geometry parameters of (a) ten lowest transition structures and (b) six products related to H2-eliminations from G2n (n = 1 → 7). Bond distances are in angstroms, and bond angles are in degrees.

The hydrogen release mechanism in these species is again characterized by the presence of short dihydrogen bonds in which the H−H distance is nearly close to that of molecular hydrogen (Figure 6a). All the corresponding TSs have structural strain due to the single or multiple four-membered rings. If we consider that the four-membered ring having H−H contact, as in G27-TS21, has the least strain, the energetics of the other TSs can readily be explained. Such an explanation remains valid even for the energy ordering of the products. The only exception to this explanation is the case of the TS G21-TS21, in which the strain is imparted because of the presence of two three-membered rings

Figure 5b illustrates the potential energy profiles describing H2-elimination reactions from G22, G24, G25, G26, and G27. Unlike the reactions in Figure 5a, it shows that the reactants eliminate H2 with different end products. The hydrogen departure from G27 is found to be exothermic (end product G33), while all the other reactions are now endothermic. The TS from G27 (G27TS21) actually has the lowest barrier height of 14.8 kcal/mol. The highest barrier height of 43.1 kcal/mol is observed for the path from G24 through G24-TS23. The rest of the TSs fall between these values (G25-TS21 from G25, 41.3 kcal/mol; G21-TS21 from G21, 39.9 kcal/mol; G26-TS21 from G26, 29.9 kcal/mol). 4533


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and G43, respectively, as products via the TSs G33-TS23 and G34-TS23. As can be seen from these TSs, short dihydrogen bonds are invariably involved. The TS G31-TS11 has the highest energy inducing the largest energy barrier of 37.8 kcal/mol. The lowest energy barrier of 8.8 kcal/mol is linked to G36-TS31 (Figure 7). The barrier heights determined by the location of the rest of the TSs are lying between these two limiting barriers. Considering the relative ring strain (as discussed earlier) and the nature of dihydrogen bond interactions, the relative ordering of the computed barrier heights can be understood. The product G41 is in fact an amino-substituted derivative of the aminodialane. G42 is a substituted structure of Cyc [AlH2NH2]2, whereas G43 is a six-membered ring of C4v symmetry. Energetically, E(G41) < E(G42) < E(G43), and such an ordering of the products is in part reflected in the molecular strain of the corresponding molecules. 3.1.6. Alazine: Formation Followed by the Fifth H2-Release. From the fourth H2-losses, three products, G41 + H2, G42 + H2, and G43 + H2, are now considered to be the starting reactants for the fifth dehydrogenation. From G42, three TSs for H2 elimination are G42-TS11, G42-TS12, and G42-TS22. The energy barriers for H2-elimination via G42-TS11, G42-TS12, and G42-TS22 are however very large (∼89, 97, and 76 kcal/mol at the MP2/aVDZ + ZPE, respectively), and these reactions are not discussed further. Selected optimized geometrical parameters of these TSs are gathered in the Supporting Information. Figure 9 shows the energy profile for H2 eliminations from the two starting compounds G41 and G43. Both G41 and G43 are found to generate the same final product, G51, via different transition states. G43 releases H2 through G43-TS and with a large energy barrier of ∼55 kcal/mol (MP2/aVDZ + ZPE). Dehydrogenation from G43 proceeds through two steps: H-migration and bond dissociation. Dihydrogen interactions between the H atoms are originated from the NH2 and AlH2 groups of aminodialane moiety. Once the H2-elimination takes place, the central H atom in the Al−H−Al hydrogen bond moves to the Al atom, which has just transferred a H atom for the elimination step. This process needs some extra energy to break the Al−H−Al hydrogen bond, and thereby causes a significant increase in the energy barrier for H2-loss. The final product G51 is actually alazine AZ, with the chemical formula (Al3N3H6, a trimer of [AlNH2]3). This cyclic species has a C3v point group symmetry, the distance r(Al−N) = 1.82 Å, and the bond angles θ(N−Al−N) = 114.7° and θ(Al−N−Al) = 125.3°; the transformation producing G51 + 5H2 formation is finally endothermic with respect to G43 + 4H2 (Figure 9). The H2-elimination from G41 takes place primarily through a high energy TS G41-TS11, which gives rise to a barrier of 48.0 kcal/mol, and the TS is characterized by an imaginary frequency of 1207i cm−1. Similar to G43-TS, the dehydrogenation from G41 also requires two different movements, including a hydrogen transfer and a bond breaking. The hydrogen-transfer process is similar to that occurring in G21t-TS12, but in this case a substantial amount of energy is required to cleave the Al−H−Al hydrogen bond in order to release two H atoms from central N- and Al- atoms. As a consequence, G41-TS11 is a high-energy structure, manifested in a large energy barrier for H2 loss. The product G52 + H2 is 30.1 kcal/mol above G41 + 4H2. G52 is an isomer of G51 and an analog of borazine. G52 bears a structure with two cyclic moieties (central r(Al−N) = 2.14 Å, the rest of r(Al−N) = 1.79 Å) and suffers higher strain than G51 and consequently higher energy. As a result, a final isomerization

Table 3. Relative Energies of the Reactants, Complexes, Transition States, and Products of the Third H2-Loss Processes relative energy (including ZPE corrections) (kcal/mol) species reactants G21t G22 G24 G25 G26 G27 TSs G21t-TS11 G21t-TS23 G22-TS13 G24-TS23 G24-TS12 G25-TS21 G25-TS12 G26-TS23 G26-TS21 G27-TS21 products G31 G32 G33 G34 G35 G36

MP2/aVDZ

CCSD(T)/aVDZ

CCSD(T)/aVTZ

0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0

40.6 38.3 30.2 29.0 41.8 28.7 41.1 7.7 29.5 14.8

42.6 40.5 31.8 31.1 44.3 30.5 43.4 9.1 31.4 15.8

39.9 38.8 29.5 29.6 43.1 28.9 41.1 8.3 30.3 14.9

0.1 −12.9 7.4 19.2 5.8 21.5

2.3 −11.4 9.8 21.6 8.1 24.4

0.1 −13.5 7.3 16.9 3.4 21.2

(formed from short dihydrogen bonds, Figure 6a). This strain, which is naturally much higher than four-membered ring strain found in G27-TS21 (Figure 6a) and several similar cases, makes G21-TS21, among others, a high-energy TS. The major outcome of the H2-elimination reactions is that ten of the located TSs from G21, G22, G24, G25, G26, and G27, the TS G26-TS23 (located at −21.6 kcal/mol) is associated with the lowest energy barrier of 8.3 kcal/mol, and the energy barriers of the rest of the TSs are found in the range of 8.3−43.1 kcal/mol. All the dehydrogenation processes considered (products delivered from G31 to G36) are endothermic, except for the products from G26 to G31, and G27. The product G31 is not only the lowest-energy product identified in this series of reactions, but it also is formed from six dehydrogenation processes involving the TSs G21-TS23, G22-TS13, G24-TS23, G25-TS21, and G26-TS23. It apparently turns out to be the most attractive and promising aspect for a continuous ammonia alane-based dehydrogenation processes. 3.1.5. The Fourth H2 Elimination: Novel Cyclic Form of H2Al[HNAlH]2NH2. The starting reactants G31−G36 are the end products from the last series of reactions as discussed above. The schematic energy profiles for the reaction paths are depicted in Figure 7, and selected geometric features of the associated TSs and products are displayed in Figure 8. The H2-elimination reactions from these starting compounds give rise to three products, namely, G41, G42, and G43, and the TSs involved are all different from each other. G41 is produced from the products G31, G33, G35, and G36 via TSs G31-TS11, G33-TS23, G35TS23, and G36-TS31, respectively. The reaction is endothermic for G31 and exothermic for the remaining H2-eliminations (Figure 7). The dehydrogenations of G33 and G34 generate G42 4534


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Figure 7. Potential energy surfaces (PES) of H2-release from G3n (n = 1 → 6). Relative energies in kilocalories per mole are from CCSD(T)/aug-ccpVTZ + ZPE calculations.

releases derived from the reaction of ammonia alane with AlH3NH2AlH2NH3 (Gau). Our calculated results suggest that in most cases, the H2‑losses are most likely to occur with a consistent assistance of dihydrogen bond interactions. The intermediate structures formed through H2 release from a step can then play a role either as reactant compounds for the subsequent step of H2 release or as catalysts for the same process. The series of products from ammonia alane starting materials [HAlNH]3 can be delivered in overcoming rather small energy barriers. It appears from the present extensive survey that although a series of reactions are involved, not all of them are viable for H2 release. Overall, from a theoretical point of view, the present study demonstrated the double role of ammonia alane, playing the role not only of a starting material but also of an efficient catalyst in a key step for the formation of a series of aluminum−nitrogen−hydrogen compounds. Such a catalytic effect of ammonia alane is clearly proved by a substantial reduction of the corresponding energy barrier and its regeneration in the products. The successive dehydrogenation processes create several novel intermediate structures, which are important in the long transformation of the starting ammonia alane-based materials. This study thus established the link between the H2-releasing molecules and the formation of (AlNH)n oligomers and

reaction is expected via TS-G51−2 (having an imaginary frequency of 192i cm−1; the central r(Al−N) being 2.65 Å, Figure 9) with a very small energy barrier of ∼1 kcal/mol with respect to G52 to produce G51. The overall formation of G51 from G41 remains endothermic. In summary, the potential energy profiles describing the dehydrogenation processes from the starting reactants AAl + Gau have been carefully explored by considering the H2-loss in each step. A number of interesting and important points which emerge from the dehydrogenation pathways are as follows. (i) Five possible complexes are formed during the AAl + Gau condensation. Only two complexes, G01 and G02, ultimately lead to formation of the cyclic products CTA, AZ, and other important intermediates. (ii) In spite of extensive attempts, we are not able to find any pathway for the formation of CTA or AZ from the higher-energy complexes G03, G04, and G05. An overview picture of the dehydrogenation processes from the starting reactants AAl + Gau is displayed in Figure 10. It is a helpful summary to understand the long chain of reactions studied in the present work.

4. CONCLUDING REMARKS We have performed a systematic exploration of the potential energy surfaces associated with the successive hydrogen 4535


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Figure 8. Selected MP2/aug-cc-pVDZ geometry parameters of seven lowest transition structures and three products related to H2-eliminations from G3n (n = 1 → 6). Bond distances are in angstroms, and bond angles are in degrees.

Figure 9. Potential energy surfaces of the formation of alazine through the H2-releases from G41 and G43 (n = 1 → 6). Relative energies in kilocalories per mole are from CCSD(T)/aug-cc-pVTZ + ZPE calculations.

identifies the molecular mechanism leading to cyclotrialazane and alazine, the six-membered cycles analogous to borazine, that

likely behave as key intermediates in the formation of (AlN)n product polymers. It is to be noted further that the bifunctional 4536


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Figure 10. Overview of the potential energy surface of the molecular system AlH3NH3 (AAl) + BH3NH2BH2NH3 (Gau). Relative energies in kilocalories per mole are from CCSD(T)/aug-cc-pVTZ + ZPE calculations.

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catalytic effect discussed in this work is one of the crucial points which has been demonstrated using extensive theoretical results. Thus, we have (i) elucidated the intriguing molecular mechanisms for hydrogen release with reasonably high accuracy and (ii) contributed to a (partial) resolution of one of the major current issues in chemical hydrogen storage materials regarding the capacity of compounds based on ammonia alane. Attempts for the preparation and determination of the electronic and molecular properties of these cyclic molecules, alazine in particular, are highly desirable.

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(1) George, T.; Parks, G. Potential Roles of Ammonia in a Hydrogen Economy: A Study of Issues Related to the Use of Ammonia for On-Board Vehicular Hydrogen Storage; U.S. Department of Energy: Washington, DC, 2006 (2) Klerke, A.; Christensen, C. H.; Norskov, J. K.; Vegge, T. Ammonia for Hydrogen Storage: Challenges and Opportunities. J. Mater. Chem. 2008, 18, 2304−2310. (3) Lipman, T.; Shah, N. Ammonia as an Alternative Energy Storage Medium for Hydrogen Fuel Cells: Scientific and Technical Review for NearTerm Stationary Power Demonstration Projects, Final Report; UC Berkeley Transportation Sustainability Research Center: Berkley, CA, November 1, 2007. (4) Christensen, C. H.; Johannessen, T.; Sorensen, R. Z. Towards an Ammonia-Mediated Hydrogen Economy? Catal. Today 2006, 111, 140−144. (5) Zamfirescu, C.; Dincer, I. Using Ammonia as a Sustainable Fuel. J. Power Sources 2008, 185, 459−465. (6) Gutowski, M.; Autrey, T. Hydrogen Gets Onboard. Chem. World 2006, 3 (3). (7) Noorden, R. V. Hydrogen Storage Targets Out of Reach Chem. World 2007, 4 (10). (8) Noorden, R. V. Tantalising Boost for Hydrogen Storage. Chem. World 2007, 4 (11). (9) Evans, J. Focus on Palladium’s Hydrogen Storage Potential. Chem. World 2005, 2 (November 29). (10) Crombie, J. Hydrogen Storage Steps Up a Gear. http://www.rsc. org/Publishing/ChemScience/Volume/2009/04/Hydrogen_storage_ steps_up.asp, March 2009. (11) Chapman, V. Borane Leads the Way to Alternative Fuels: Fuelling the Future. Chem. Sci. 2008, 5, C59. (12) Crow, J. M. The Holey Grail of Hydrogen Storage. Chem. Technol. 2008, 5, T3.

ASSOCIATED CONTENT

S Supporting Information *

Tables listing the Cartesian coordinates of the structures considered, total electronic energies, and thermochemical parameters. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.T.N. is indebted to the KU Leuven Research Council for continuing support within the GOA and PDM programs. This work was supported by the PREM (Award DMR-1205194) and ONR (Award N00014-13-1-0501) grants. 4537


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Hydrogen Release from Ammonia Alane-Based Materials: Formation

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