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J. Phys. Chem. C 2009, 113, 18914–18926
Computational Study of Molecular Complexes Based on Ammonia Alane for Chemical Hydrogen Storage Vinh Son Nguyen,† Saartje Swinnen,† Minh Tho Nguyen,*,†,‡ and David A. Dixon*,‡ Department of Chemistry, The UniVersity of Alabama, Shelby Hall, Tuscaloosa, Alabama 35487-0336, and Department of Chemistry, and Mathematical Modeling and Computational Science Center (LMCC), Katholieke UniVersiteit LeuVen, B-3001 LeuVen, Belgium ReceiVed: May 10, 2009; ReVised Manuscript ReceiVed: August 12, 2009
Electronic structure calculations at the CCSD(T) level with the aug-cc-pVnZ and aug-cc-pV(n+d)Z basis sets (n ) D, T, and Q) were employed to construct the potential energy surfaces for H2 release from a series of derivatives of ammonia alane. H2 production from AlH3NH3 is facilitated by the addition of alane, which plays the role of a Lewis acid bifunctional catalyst. H2 release is not competitive from the combination of two AlH3NH3 monomers in the form of a stable dimer. An alternative route involves the reactions of one AlH3NH3 monomer with two separated molecules of AlH3 and NH3, in two successive steps: (i) an initial condensation of AlH3NH3 with NH3 leads to a stable linear NH3AlH3NH3 species, and (ii) a subsequent combination of the latter with AlH3 gives rise to a bicycle framed by dihydrogen bonds. From the new bicyclic form, H2 production becomes a facile process. Condensation of dialane (Al2H6) with two separated ammonia molecules (2NH3) is also a potential route for H2 release. The reaction enthalpies and free energies at 298 K were also evaluated for the pathways considered. Although the effects of temperature and entropy are important, the ∆G298 preferred pathways remain similar to the energy (∆H(0 K)) profiles. Introduction In the emerging hydrogen economy, hydrogen storage remains one of the most critical issues that must be addressed for hydrogen to be a practical fuel for use in the transportation sector.1 Extensive efforts are underway to design and synthesize materials for efficient chemical H2 storage. With a low molecular weight (30.9 g mol-1) and a high potential H2 capacity (19.6 wt %), in conjunction with both the acidic NH and basic BH bonds, ammonia borane (BH3NH3, ab) and its derivatives are currently being considered as hydrogen sources.2 In recent theoretical studies,3-10 our computational efforts have been focused on the determination, using high-accuracy quantum chemical calculations, of the thermochemical and kinetic parameters for H2 release from the smallest BNH derivatives, including BH3NH3, B2H6NH3, B3H7NH3, (BH3NH3)2, (BH2NH2)n, BH3N2H4, and B2H6N2H4. We have predicted, among other things, a strong effect of the Lewis acid borane on H2 release, which acts as a bifunctional catalyst involving H transfer within a cyclic framework with a N-Hδ+-Hδ--B type of dihydrogen bond. There has been considerable interest in metal hydrides for hydrogen storage,11 including aluminum-based materials such as NaAlH4.12,13 The amine complexes of aluminum hydrides had first been prepared in the 1960s by different groups.14,15 Alane (AlH3)-based compounds have been suggested as a promising material for onboard hydrogen storage applications.11,16,17 Issues with all hydrogen storage materials for transportation applications are dehydrogenation and rehydrogenation of the spent fuel, both under appropriate conditions. Reactions of mixed aluminum hydride derivatives with * To whom correspondence should be addressed. E-mail: minh.nguyen@ chem.kuleuven.be (M.T.N.);
[email protected] (D.A.D.). † Katholieke Universiteit Leuven. ‡ The University of Alabama.
ammonia borane lead to loss of H2 and formation of AlN/ BN polymeric materials at high temperatures.18 We predicted that gaseous ammonia alane (AlH3NH3, aal) and its ionic salt [AlH4-][NH4+] in the solid state could serve as good hydrogen storage systems that release H2 in near-thermoneutral processes.9 The thermodynamics of the isovalent phosphorus systems including borane phosphine (BH3PH3) and phosphine alane (AlH3PH3), and their corresponding crystal salts [XH4-][PH4+], with X ) B and Al, were also predicted in our studies of potential H2 storage systems.9,10 Recently, we found that alane (AlH3) plays the role of a bifunctional catalyst, exerting a larger catalytic effect than borane for H2 production either from C2H6, BH3NH3, or AlH3NH3.19 On the basis of our previous theoretical studies,19 Al-N-H compounds potentially have good thermodynamics and kinetics for H2 release and spent fuel regeneration. We have extended the scope of our prior work on AlH3 + AlH3NH3 using high-level molecular orbital theory to predict the energetics of the molecular mechanism for H2 release from a series of complexes of alane and ammonia. The systems considered in this work include RNH2AlH3 (R ) H, CH3, NH2), NH3(AlH3)2, AlH3(NH3)2, and several forms of the dimers (AlH3NH3)2. Computational Methods Molecular orbital theory calculations were carried out by using the Gaussian 0320 and Molpro 200621 suites of programs. We used the augmented correlation consistent (aug-cc-pVnZ) basis sets.22 For the sake of brevity, we abbreviate the basis sets to aVnZ, with n ) D, T, and Q. For all structures, geometries and harmonic vibrational frequencies were initially determined using second-order perturbation theory (MP2)23 with the aVDZ basis set. Geometries of the important equilibrium and transition state (TS) structures were subsequently refined at the MP2/aVTZ level. Relative energies were then calcu-
10.1021/jp904344p CCC: $40.75 2009 American Chemical Society Published on Web 10/05/2009
Computational Study of Molecular Complexes
J. Phys. Chem. C, Vol. 113, No. 43, 2009 18915
TABLE 1: Reaction Enthalpies at 0 and 298 K and Gibbs Energies at 298 K of the Reactants, Complexes, Transition States, and Products in the H2 Elimination Processes from the Linear Structure NH3AlH3NH3 + AlH3 and Al2H6NH3 + NH3 in kcal/mola structure
∆H0
∆H298
∆G298
2AlH3NH3 (2ala) NH3AlH3NH3 + AlH3 (daal-lin + al) Al2H6NH3 + NH3 (dala-lin + a) Al2H6 + 2NH3 (dal + 2a) AlH3AlH2NH3NH2 + H2 AlH3NH2AlH2NH3 + H2 AlH2NH2AlH2NH2 + 2H2 dalda-lin dadal-lin DADAl ts-2lin dalda-lin-ts dadal-lin-ts ts-lin-DADAl DADAl-ts AlH3NH2AlH2NH3-ts + H2
0.0 15.6 2.9 18.6 -14.0 -29.1 -46.5 -0.6 -11.6 -4.1 1.4 15.3 17.2 -2.2 4.0 -10.6
0.0 16.2 3.2 19.5 -13.3 -29.4 -45.5 -0.7 -11.9 -4.9 1.2 14.9 16.1 -3.5 3.5 -10.5
0.0 14.9 3.6 10.8 -11.1 -25.3 -49.1 8.2 -1.7 6.1 10.9 27.1 27.9 8.4 12.8 -7.3
a Relative values obtained from total energies at the CCSD(T)/ aVTZ level. Energy values, ZPE, thermal corrections, and entropies are listed in Table S7 (SI). The ∆H0 profiles are displayed in Figure 8 and the ∆G298 profiles in Figure S5 (ESI).
TABLE 2: Reaction Enthalpies at 0 and 298 K and Gibbs Free Energies at 298 K of the Reactants, Complexes, Transition States, and Products in the H2 Elimination Processes from the Bent Structures NH3-NH3AlH3 + AlH3 and H3AlHAlH2NH3 + NH3 in kcal/mola structure
∆H0
∆H298
∆G298
2AlH3NH3 (2ala) dim1-ala dalda-trans IonP-ala AlH3NH2AlH2NH3 + H2 AlH2NH2AlH2NH2 + H2 IonP-ala-ts ts-trans-IonP ts-dim1-IonP AlH3NH2AlH2NH3-ts + H2
0.0 -13.1 -7.3 2.1 -29.1 -46.5 5.1 28.4 19.9 -10.6
0.0 -13.2 -6.9 1.2 -29.4 -45.5 4.3 27.8 19.5 -10.5
0.0 -4.2 -0.6 12.5 -25.3 -49.1 15.0 37.0 29.3 -7.3
a Relative values obtained from CCSD(T)/aVTZ total energies. Energy values, ZPE, thermal corrections, and entropies are listed in Table S8 (SI). The ∆H0 profiles are displayed in Figure 11 and the ∆G298 profiles in Figure S6 (SI).
lated at the coupled-cluster theory CCSD(T)24,25 level at the MP2/aVTZ geometries with the aVnZ basis sets. Zero-point vibrational energies (ZPE) were obtained from MP2/aVDZ frequencies and were scaled by a factor of 0.97 as described in previous studies.3,9 In addition to the above calculations, we also studied RNH2AlH3 (R ) H, CH3, and NH2) at a higher level. It has been established that tight d-functions can be necessary for calculating accurate atomization energies for second-row elements.9,26 Thus, we included a set of tight d-functions for Al in our calculations. Basis sets containing additional tight d-functions are denoted as aug-cc-pV(n+d)Z or in a simpler notation as aV(n+d)Z. For the reaction pathways of the RNH2AlH3 (R ) H, CH3, and NH2) systems, geometry parameters were reoptimized using the aV(T+d)Z basis set. CCSD(T) single-point electronic energy calculations were also carried out using the aV(n+d)Z basis set at the MP2/ aV(T+d)Z geometries. The final total valence electronic energies were then extrapolated to the complete basis set
TABLE 3: Reaction Enthalpies at 0 and 298 K and Gibbs Free Energies at 298 K of the Reactants, Complexes, Transition States, and Products in the H2 Eliminations from the Dimer Structures 2(AlH3NH3) in kcal/mola structure
∆H0
∆H298
∆G298
2AlH3NH3 (2ala) Al2H6 + 2NH3 (dal + 2a) dim1-ala dim2-ala dim3-ala dim4-ala NH2AlH2HAlH2NH3 + H2 AlH3NH2AlH2NH3 + H2 AlH2NH2AlH2NH2 + H2 NH2AlH2AlH3NH3 + H2 2ala-ts dim1-ala-tsmono dim1-ala-ts dim2-ala-ts dim2-ala-ts2H2 dim3-ala-ts1 dim4-ala-ts1 AlH3NH2AlH2NH3-ts + H2
0.0 18.6 -13.1 -12.2 -4.3 -9.6 -12.1 -29.1 -46.5 -11.8 25.5 18.4 19.2 19.8 22.2 20.7 18.2 -10.6
0.0 19.5 -13.2 -12.4 -9.8 -4.5 -11.1 -29.4 -45.5 -10.8 25.0 17.6 18.1 19.6 21.3 20.1 17.4 -10.5
0.0 10.8 -4.2 -3.6 -0.2 5.1 -10.6 -25.3 -49.1 -9.3 34.5 28.1 29.6 28.1 32.0 30.3 28.0 -7.3
a Relative values are given in kcal/mol and obtained from CCSD(T)/ aVTZ total energies. Energy values, ZPE, thermal corrections, and entropies are listed in Table S9 (SI). The ∆H0 profiles are displayed in Figure 12 and the ∆G298 profiles in Figure S7 (SI).
(CBS) limit at the CCSD(T) level using the following expression (eq 1)27
E(n) ) ACBS + B exp[-(n - 1)] + C exp[-(n-1)2]
(1) with n ) 2, 3, and 4 for the aVnZ and aV(n+d)Z, n ) D, T, and Q, basis sets, respectively. Results and Discussion Details of the calculated results are given in the Supporting Information (SI). Total and relative energies and related thermochemical data of the optimized structures are listed in Tables S1-S9 (S stands for SI). Tables S1 and S2 list the relative and total energies calculated at the CCSD(T) level of theory using different basis sets, and the zero-point energies (ZPE) at both MP2/aVDZ and MP2/aVTZ levels for the substituted RNH2AlH3 derivatives, with R ) H, CH3, and NH2. Tables S3 and S4 give these quantities for the NH3AlH3 + NH3 system. Tables S5 and S6 list the same quantities for the NH3AlH3 + AlH3 system. Tables S7-S9 tabulate the CCSD(T)/ aVTZ total energies, ZPE, thermal corrections to the enthalpies (H298-H0) and entropies (S), as well as the T1 diagnostic values28 of the coupled-cluster CCSD wave functions, for the stationary points on the different pathways related to the dimeric system 2(AlH3NH3). Figures S1, S2, and S4 of the SI display the selected geometry parameters of the structures that are either present on the potential energy surfaces or discussed in the text but not shown. Figure S3 shows the predicted IR spectra of three dimeric forms of ammonia alane. The effects of the temperature and entropy on the course of reactions were also calculated. For each energy profile, the corresponding free energy (∆G) profile was constructed. The ∆G(298) profiles discussed are shown in Figures S5-S10 of the SI. For the purpose of detailed comparison, Tables 1, 2, and 3 record the relative enthalpies (∆H) at both 0 and 298 K and free energies (∆G) at 298 K, calculated using the CCSD(T)/
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Nguyen et al.
Figure 1. Schematic minimum energy pathways for H2 release and Al-N bond dissociation from aal and substituted alhyz and alma. Relative energies in kcal/mol from calculations at the CCSD(T)/aVTZ (upper) and CCSD(T)/CBS-aVnZ (lower) levels. All values corrected for zero-point energies. Selected geometry parameters of the transition states aal-ts, alhyz-ts, and alma-ts at the MP2/aug-cc-pVTZ level. Bond distances in Å and bond angles in degrees. Blue profile is for R) CH3, black for R ) NH2, and red for R ) H.
aVTZ total energies for the different portions of the (Al2N2H12) potential energy surface. These relative quantities are derived from calculated data tabulated in Tables S7, S8, and S9. The calculated results obtained in this work are summarized in Figures 1-12 displayed below. The T1 diagnostic values28 for the coupled-cluster CCSD wave functions listed in Tables S7-S9 are
