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Reaction mechanism of hydrogenation of carbon dioxide to formic acid in the presence of scandium oxide: a density functional study
Chemical Physics Letters 396 (2004) 75–82 www.elsevier.com/locate/cplett
Reaction mechanism of hydrogenation of carbon dioxide to formic acid in the presence of scandium oxide: a density functional study Der-Yan Hwang b
a,*
, Alexander M. Mebel
*,b
a Department of Chemistry, Tamkang University, In-tran Rd. 151, Tamsui 25137, Taiwan Department of Chemistry and Biochemistry, Florida International University, Miami, FL 33199, USA
Received 13 July 2004; in final form 13 July 2004
Abstract Density functional calculations of potential energy surfaces in the CO2/H2/ScO(2R+) system have been performed to investigate the reaction mechanism of CO2 hydrogenation to formic acid in the presence of ScO. The results show that ScO can easily form a variety of complexes with CO2 and H2, complexes of CO2 with HScOH, and a highly exothermic cyc-OC(H)OScOH molecule. Although transformation of the latter to ScO + HCOOH is impeded by the high barrier for hydrogen transfer from ScOH to HCO2, it is expected to be fast in the gas phase because the transition state is only 2.3 kcal/mol higher in energy than the reactants CO2 + H2 + ScO. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction Fixation of carbon dioxide has received much attention in recent years [1–6] due to environmental importance of CO2 as a greenhouse gas and its large-scale availability at low cost. CO2 is a highly oxidized and thermodynamically stable compound, which has low reactivity. Therefore, its utilization remains an important problem. Because of the high stability of CO2 and its inertness in chemical reactions, the toxic carbon monoxide instead of CO2 is mostly used currently for many processes in industry as a C1 building unit [6]. On the other hand, a promising approach for CO2 fixation is the use of homogeneous, heterogeneous, or enzymatic catalysis. The homogeneous catalysis for fixation of carbon dioxide has been discussed in recent reviews [2,4]. For instance, catalytic hydrogenation of *
Corresponding author. Fax: +886 2 2620 9924. E-mail address: [email protected] (D.-Y. Hwang).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.07.113
CO2 to formic acid, which provides a prospective technique allowing to use CO2 as a raw material in largescale chemical synthesis, has attracted much interest in recent years. Rhodium and ruthenium phosphine complexes were found to serve as efficient catalysts for the formation of formic acid from CO2 [7–9]. Alternatively, heterogeneous catalysis can provide several technical advantages due to stability, separation, handling and reuse of the catalyst and reactor design. However, at present, the range of compounds that have been synthesized from CO2 by heterogeneous catalytic methods is still relatively narrow and is mainly confined to methanol synthesis, the syntheses of methylamines and formic acid derivatives, and the production of synthesis gas [6]. For example, a ruthenium-containing silica hybrid catalyst has been suggested for the production of various derivatives of formic acid [6]. The CO2 + H2 ! HCOOH reaction is known to have a very high barrier without a catalyst [10–15]. For
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instance, the hydrogenation reaction of carbon dioxide occurs in two steps, H2 addition to CO2 to form cisHCOOH followed by cis–trans isomerization to produce the more stable trans-HCOOH structure. The highest barrier on this pathway is found for the initial H2 addition step and it is calculated to be 76.3 and 73.8 kcal/mol at the G2 [15] and B3LYP/6-311 + G(3df,2p)//B3LYP/ 6-31G(d,p) levels, respectively, and 70.2–73.2 kcal/mol according to experimental measurements [11,12]. In this view, a search of new efficient catalysts for the fixation of carbon dioxide by H2 to produce formic acid remains an active area of research. Metals and their compounds can catalyze many important chemical reactions, either homogeneously or heterogeneously. Transition metal oxides can coordinate CO2 [16–18] and also react with molecular hydrogen [19–21]. Therefore, it would be reasonable to expect that these oxides can assist the conversion of CO2 to formic acid, i.e., catalyze the CO2 + H2 ! HCOOH reaction. In the present work, we use theoretical density functional calculations to investigate the reaction mechanism of carbon dioxide with H2 catalyzed by ScO. Our recent study of the CO/H2/BeO system demonstrated that the addition of molecular hydrogen to carbon monoxide can be significantly enhanced in the presence of BeO [22]. In a similar way, we can expect that the presence of a metal oxide, ScO in this case, can facilitate the addition of H2 to CO2 producing formic acid. Scandium oxide reacts with molecular hydrogen producing HScOH [19,23]. The latter may then react with CO2 and the metal-oxide-bonded H atoms of HScOH can be eventually transferred to the carbon dioxide fragment. Experimentally, ScO has been thoroughly studied in the gas phase and matrix probably due to its appearance in the spectra of stars (see, for example [24–27]). As the lightest among transition metals, Sc is also very convenient for ab initio calculations, and a number of theoretical works have been devoted to ScO and its reactions [19,23,28–31]. However, the interaction mechanism of scandium oxide with the CO2/H2 mixture was not known so far and in this Letter we consider the CO2 + H2 + ScO reaction. We report potential energy surface (PES) for this reaction, elucidate the gas-phase reaction mechanism and provide geometric structures of various reactants, products, intermediates, and transition states as well as their reliable energies. This work continues our systematic studies of reaction mechanisms for various chemical reactions, including conversion of methane to methanol [32], nitrogen hydrogenation [33,34], and conversion of CO to formaldehyde [22], in the presence of metal oxides. A detailed understanding of elementary reaction steps from reactants to desired product and a comparison of the catalytic role of different transition metal oxides would be helpful to facilitate a rational design of potential catalysts.
2. Computational details Full geometry optimizations were carried out at the B3LYP/6-31G(d,p) level of theory to locate various stationary points (reactants, intermediates, transition states, and products) on the ground doublet electronic state PES of the CO2/H2/ScO system. Harmonic vibrational frequencies were calculated at the same B3LYP/6-31G(d,p) level in order to characterize the stationary points as minima or transition states, to obtain zero-point vibrational energy corrections (ZPE), and to generate force constants needed for intrinsic reaction coordinate (IRC) calculations, which were employed to confirm connections of transition states to certain local minima. The relative energies were refined using single-point calculations with B3LYP/6-31G(d,p) optimized geometry employing the B3LYP method with the larger 6-311 + G(3df,2p) basis set. All density functional calculations described here were performed employing the GAUSSIAN 98 program [35].
3. Results and discussion Here we consider the lowest in energy doublet electronic state for the CO2 + H2 + ScO(2R+) reaction. The total energies, ZPE, and relative energies of various reactants, products, intermediates, and transition states for this reaction are presented in Table 1, their vibrational frequencies are shown in Table 2. The potential energy diagram along the CO2 + H2 + ScO(2R+) ! HCOOH + ScO(2R+) reaction pathway is depicted in Fig. 1 and optimized geometries of the intermediates and transition states are illustrated in Fig. 2. 3.1. Formation of CO2–ScO–H2 and CO2–HScOH complexes There are two different possibilities for initiation of the CO2 + H2 + ScO(2R+) reaction. Scandium oxide can react first either with H2 or with carbon dioxide. We first consider the coordination of carbon dioxide to ScO. The two can form a relatively weak end-on g1-CO2–ScO complex without a barrier. The binding energy of the complex is calculated to be 3.6 kcal/mol. The geometry of the CO2 fragment in the complex changes only slightly; the coordinated C–O bond elongates by ˚ as compared to that in isolated CO2 and the 0.02 A O–C–O angle is close to 180°. The only notable change in vibrational frequencies in g1-CO2–ScO as compared to isolated CO2 and ScO is a decrease of the degenerate O–C–O bending frequency from 666 cm1 in carbon dioxide to 561 and 564 cm1 in the complex (see Table 2). The interaction between two molecular fragments in g1-CO2–ScO is described in terms of donation of electron density from the occupied pp orbital of CO2 to
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Table 1 Total energies (hartree), ZPE, and relative energies (kcal/mol) of various species in the CO2 + H2 + ScO reaction calculated at the B3LYP/6-31G(d,p) and B3LYP/6-311 + G(3df,2p)//B3LYP/6-31G(d,p) levels of theory Species
H2 ScO(2R+) CO2 CO2 + ScO(2R+) H2 + ScO(2R+) CO2 + H2 + ScO(2R+) g1-CO2–ScO(2A 0 ) TS1 g2-CO2–ScO g2-CO2–ScO + H2 g2-CO2–ScO–H2 TS2 g2-CO2–HScOH TS3(2A 0 ) HScOH(2A 0 ) CO2 + HScOH(2A 0 ) g1-CO2–HScOH TS4 TS5 OC(H)OScOH(2A 0 ) TS6(2A 0 ) cyc-OC(H)OScOH(2A 0 ) TS7 cyc-HCOO(H)ScO TS8 HCO(H)OScO HCOOH HCOOH + ScO(2R+)
B3LYP/6-31G**
B3LYP/6-311 + G(3df,2p)
Total energy
ZPE
1.17854 835.89460 188.58094 1024.47554 837.07314 1025.65408 1024.48820 1024.48638 1024.49827 1025.67681 1025.68510 1025.67470 1025.72165 837.05518 837.09746 1025.67840 1025.71100 1025.69222 1025.69884 1025.75111 1025.75031 1025.77566 1025.66796 1025.69338 1025.67791 1025.68441 189.76222 1025.65682
6.39 1.50 7.28 8.77 7.88 15.16 9.02 8.03 8.55 14.93 17.84 16.91 18.01 9.51 10.36 17.57 17.45 17.57 18.20 22.23 22.15 22.51 20.02 22.21 22.28 22.72 21.33 22.83
Relative energy
0 0 0 7.69 7.55 14.49 14.49 16.78 11.19 39.55 12.90 12.78 12.78 33.42 21.52 25.04 53.82 53.39 68.94 3.85 17.60 7.83 11.47 5.95
Total energy 1.18001 835.96909 188.65985 1024.62894 837.14910 1025.80895 1024.63505 1024.63217 1024.64479 1025.82480 1025.83345 1025.82189 1025.87123 837.12984 837.17561 1025.83546 1025.86154 1025.84674 1025.85226 1025.90242 1025.89920 1025.92742 1025.81308 1025.83994 1025.82252 1025.83395 189.84022 1025.80931
Relative energy
0 0 0 3.58 2.77 10.17 10.17 12.69 6.37 36.23 13.69 14.18 14.18 30.71 21.30 24.14 51.58 49.64 66.99 2.27 12.39 1.40 8.13 7.45
Table 2 Vibrational frequencies (cm1) of various compounds in the CO2 + H2 + ScO reaction calculated at the B3LYP/6-31G(d,p) level Species 1
g -CO2–ScO TS1 g2-CO2–ScO g2-CO2–ScO–H2 TS2 g2-CO2–HScOH TS3 HScOH g1-CO2–HScOH TS4 TS5 OC(H)OScOH TS6 cyc-OC(H)OScOH TS7 cyc-HCOO(H)ScO TS8 HCO(H)OScO
Frequencies 26, 37, 103, 178, 561, 564, 1032, 1370, 2441 280i, 25, 103, 131, 366, 395, 1031, 1238, 2324 70, 153, 197, 348, 404, 689, 1020, 1175, 1922 40, 73, 145, 198, 211, 349, 398, 484, 678, 698, 981, 1011, 1184, 1915, 4115 1191i, 73, 98, 163, 204, 358, 418, 703, 886, 952, 1180, 1271, 1703, 1899, 1918 74, 129, 179, 221, 231, 358, 373, 405, 507, 711, 795, 1173, 1563, 1922, 3964 1364i, 885, 949, 1256, 1649, 1911 302, 327, 342, 764, 1523, 3944 45, 70, 101, 143, 162, 248, 384, 395, 528, 742, 803, 1191, 1570, 1864, 3959 54i, 33, 105, 156, 183, 279, 295, 371, 477, 514, 735, 1316, 1468, 2412, 3949 764i, 78, 130, 294, 315, 386, 391, 462, 692, 732, 758, 1093, 1506, 1928, 3962 34, 47, 121, 139, 365, 368, 504, 743, 775, 1054, 1240, 1413, 1801, 2992, 3949 90i, 30, 84, 170, 375, 388, 519, 743, 774, 1060, 1251, 1403, 1757, 3009, 3939 38, 60, 235, 326, 358, 376, 379, 739, 818, 1060, 1351, 1389, 1591, 3057, 3969 998i, 141, 177, 256, 297, 569, 674, 829, 938, 1090, 1255, 1357, 1389, 1913, 3134 105, 146, 183, 309, 405, 460, 616, 721, 978, 1002, 1089, 1263, 1411, 3009, 3841 111i, 59, 128, 213, 236, 423, 672, 731, 1021, 1057, 1222, 1351, 1635, 3117, 3732 66, 112, 166, 279, 446, 466, 587, 850, 997, 1061, 1271, 1289, 1462, 3084, 3764
empty d orbitals of Sc and back donation from the dr MO of ScO to the empty antibonding pr* MO of CO2 and, judging from the binding energy of g1-CO2–ScO, this interaction is rather weak. However, it is stronger than that between N2 and ScO, for which no end-on
complex was found on PES [34]. g1-CO2–ScO represents only a metastable local minimum on PES and easily rearranges to the side-on g2-CO2–ScO complex via a low barrier of 0.8 kcal/mol at TS1. The side-on complex is much more strongly bound than g1-CO2–ScO and lies
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CO2+TS3 13.7 0.0 CO2+H2+ScO(2Σ+)
η1-CO2-ScO+H2
TS1 -2.8
-3.6
TS2 -6.4
η2-CO2-ScO+H2 -12.7 -10.2
-14.2
η2-CO2-ScO-H2
CO2+HScOH
TS4 -21.3
-30.7 1
η -CO2-HScOH
-36.2 η2-CO2-HScOH
(a)
TS7 2.3
TS8 -1.4 -12.4
HCOOH+ScO(2Σ+) 7.4 -8.1 HCO(H)OScO
cyc-HCOO(H)ScO TS5 -24.1 -36.2 2
η -CO2-HScOH -51.6
TS6 -49.6
OC(H)OScOH
(b)
-67.0 cyc-OC(H)OScOH
Fig. 1. Potential energy diagram for the CO2 + H2 + ScO(2R+) ! HCOOH + ScO(2R+) reaction calculated at the B3LYP/6-311 + G(3df,2p)// B3LYP/6-31G(d,p) + ZPE[B3LYP/6-31G(d,p)] level: (a) formation of various CO2/ScO/H2 and CO2/HScOH complexes; (b) pathway leading to formic acid. Relative energies of various species are given in kcal/mol.
10.2 kcal/mol lower in energy than CO2 + ScO. The distortion of the CO2 fragment in g2-CO2–ScO is also much more significant; the coordinated C–O bond is ˚ , the other C–O bond is also 0.03 lengthened by 0.11 A ˚ longer than that in free carbon dioxide, and the A O–C–O angle decreases to 141.3°. These geometric changes are reflected in vibrational frequencies of g2CO2–ScO. For instance, the C–O asymmetric stretch frequency decreases from 2438 cm1 in CO2 to 1922 cm1 in the complex, the C–O symmetric stretch frequency changes from 1376 to 1175 cm1, and the O–C–O bending frequencies become 404 and 689 cm1. These changes are large enough to be observable in FTIR matrix isolation experiments if ScO is mixed with carbon dioxide, especially for the IR-intense C–O asymmetric stretch and the O–C–O bending vibrations. The symmetric stretch vibration, which is not IR-active in free CO2, also may become observable in the complex. The Sc–O stretching frequency is less sensitive to the complex formation; it changes only slightly from 1048 cm1 in ScO to 1020 cm1 in g2-CO2–ScO, as calculated
at the same B3LYP/6-31G(d) level of theory. The interaction between the fragments in the g2-CO2–ScO complex is similar to that in g2-N2–ScO considered earlier [34]: donation of electron density from the bonding pp MO of CO2 to the empty dr orbital of Sc accompanied with back donation from the singly occupied dd Sc orbital to the antibonding pp* MO of CO2. To make such interaction possible, the single electron from the dr orbital of scandium atom in the ground electronic state of ScO has to be promoted to the dd orbital; this causes the appearance of the barrier at TS1 between the g1- and g2-CO2–ScO complexes. g2-CO2–ScO can in turn form another, weakly bound trimolecular complex with H2, g2-CO2–ScO–H2, 2.5 and 12.7 kcal/mol lower in energy than the bimolecular sideon complex of CO2 with ScO and the initial reactants, respectively. In the complex, the H2 and CO2 ligands attached to the Sc atom lie in perpendicular planes and H2 interacts with ScO through the rg ! dr (Sc) donation. The Sc atom in g2-CO2–ScO is better prepared for this kind of interaction with molecular hydrogen as its single
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Fig. 2. Geometries of various intermediates and transition states of the CO2 + H2 + ScO(2R+) ! HCOOH + ScO(2R+) reaction optimized at the B3LYP/6-31G(d,p) level. Bond lengths are given in angstroms and bond angles in degrees.
electron is already promoted to the dd-type orbital and the dr orbital is empty. No molecular complex exists between H2 and the ground state ScO(2R+), such complex was found only for the excited 2D state of scandium oxide [34], which is formed by the dr ! dd electronic
transition. Due to coordination to the scandium atom, ˚ and the the bond length in H2 increases by 0.02 A H–H vibrational frequency changes from 4419 to 4115 cm1. However, this vibration is IR-inactive in free H2 and remains very weak in the complex. The changes of
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the C–O and Sc–O stretching and O–C–O bending frequencies from g2-CO2–ScO to g2-CO2–ScO–H2 are minor, within 10 cm1, but the vibrational spectrum of the latter is calculated to have additional peaks at 40, 211, 484, 678, and 981 cm1 (see Table 2). The g2CO2–ScO–H2 complex isomerizes to g2-CO2–HScOH by insertion of ScO into the H-H bond via transition state TS2. The barrier for this process is 6.3 kcal/mol relative to g2-CO2–ScO–H2 and TS2 lies 6.4 kcal/mol lower in energy than CO2 + H2 + ScO(2R+). The g2-CO2– HScOH intermediate is 36.2 kcal/mol more stable than the reactants. The structure and orientation of the CO2 fragment in g2-CO2–HScOH are similar to those in g2-CO2–ScO; the coordinated C@O bond is stretched ˚ as compared to the bond length in isolated by 0.12 A CO2, the second C@O bond length increases by 0.03 ˚ , and the OCO angle is 140.8°. Similar to g2-CO2– A ScO, the bonding between the fragments in g2-CO2– HScOH can be described in terms of electron donation from the bonding pp MO of CO2 to the empty dr-type orbital of Sc and back donation from a singly occupied dd-type orbital of the metal atom to the antibonding pp* MO of CO2. Earlier [34], we found a similar side-on complex of HScOH with N2, but g2-N2–HScOH is less strongly bound with respect to N2 + HScOH, by 16.8 kcal/mol as compared to 22.0 kcal/mol for g2-CO2– HScOH. Now we consider the alternative pathway leading from CO2 + H2 + ScO(2R+) to g2-CO2–HScOH starting from the studied earlier ScO + H2 reaction [19]. In the ground electronic state, scandium oxide can insert into the H–H bond via a Cs-symmetric transition state TS3(2A 0 ) with a barrier of 13.7 kcal/mol. After the insertion, the planar HScOH (3A 0 ) molecule is formed, which lies 14.2 kcal/mol lower in energy that the initial reactants. From a comparison between TS3 and TS2 (insertion transition states of isolated ScO(2R+) and the ScO fragment of g2-CO2–ScO, respectively, into H2), we can see that the latter has an earlier character than the former displayed in the shorter H–H distance for the breaking bond and longer O–H and Sc–H lengths for the forming bonds. The barrier at TS2 (with respect to the reactant at this particular reaction step) is 7.4 kcal/ mol lower than that at TS3 indicating that the insertion process becomes more facile due to the coordination of CO2 to ScO. This can be attributed to the fact that in the g2-CO2–ScO complex the single electron of Sc is promoted from the dr to dd orbital and therefore the donation of electron density from rg(H2) to dr(Sc) and back donation from dd(Sc) to ru*(H2) taking place during the insertion process occur more easily. At the next reaction step, HScOH can form a strong end-on complex with carbon dioxide, g1-CO2–HScOH, bound by 16.5 kcal/ mol relative to the separated HScOH and CO2 molecules. The complex formation occurs without a barrier and g1-CO2–HScOH lies 30.7 kcal/mol lower in energy
than CO2 + H2 + ScO(2R+). In HScOH, the unpaired electron still occupies the dr orbital of Sc and the mechanism of complex formation is similar to that for g1CO2–ScO, electron donation from the pp orbital of CO2 to empty d orbitals of Sc and back donation from the dr MO of ScO to the pr* MO of CO2. Interestingly, the end-on complex of CO2 with HScOH is stronger than with ScO(2R+); thus, the presence of one ligand, H2, increases the binding energy with the second ligand, CO2, and vice versa. g1-CO2–HScOH can rearrange to the side-on g2-CO2–HScOH complex over a 9.4 kcal/ mol barrier at TS4. As in the case of the CO2–ScO complexes, the barrier originates from the necessity to promote the single electron from the dr to dd orbital on Sc. The side-on complex is 5.5 kcal/mol more favorable than g1-CO2–HScOH. Comparing vibrational spectra of isolated HScOH + CO2 and g1-CO2–HScOH and g2-CO2–HScOH complexes, we can see a blue shift for the frequencies of HScOH (O–H stretch: from 3944 cm1 in HScOH to 3959 and 3964 cm1 in g1 and g2, respectively; Sc–H stretch: from 1523 cm1 to 1570 and 1563 cm1; and Sc–O stretch: from 764 cm1 to 803 and 795 cm1) and a red shift for the CO2 frequencies (asymmetric C–O stretch: from 2438 cm1 in CO2 to 1864 and 1922 cm1 in g1 and g2, respectively; symmetric C–O stretch: from 1376 cm1 to 1191 and 1173 cm1). A comparison of the two mechanisms for the formation of g2-CO2–HScOH shows that the ÔCO2 first, then H2Õ pathway, CO2 + H2 + ScO(2R+) ! H2 + g1-CO2– ScO ! TS1 ! H2 + g2-CO2–ScO ! g2-CO2–ScO–H2 ! TS2 ! g2-CO2–HScOH, is clearly preferable as compared to the ÔH2 first, then CO2Õ pathway, CO2 + H2 + ScO(2R+) ! TS3 ! CO2 + HScOH ! g1CO2–HScOH ! TS4 ! g2-CO2–HScOH, because for the latter the barrier of 13.7 kcal/mol relative to the initial reactants has to be overcome. For the former, owing to the barrier decrease for the insertion into the H–H bond and to the fact that the g2-CO2–ScO–H2 complex is significantly bound with respect to the initial reactants, the transition state TS2 lies 6.4 kcal/mol below CO2 + H2 + ScO(2R+). Therefore, the CO2 + H2 + ScO(2R+) reaction through the ÔCO2 first, then H2Õ mechanism is expected to be fast (all intermediates and transition states are lower in energy than the reactants) and the g2-CO2–HScOH complex can be readily produced. 3.2. Formation of formic acid The g2-CO2–HScOH complex can isomerize to the planar OC(H)OScOH intermediate by 1,3-hydrogen migration from Sc to the C atom via TS5. The barrier for this process is calculated to be 12.1 kcal/mol and the transition state resides 24.1 kcal/mol lower in energy than the initial reactants. TS5 has a four-member ring
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ScHCO fragment and an early character with the break˚ longer than that in the g2-CO2– ing Sc–H bond 0.04 A ˚ HScOH reactant and the forming C–H bond 0.63 A longer than that in the reaction product. After the barrier at TS5 is cleared, the COSc angle eventually in˚ creases from 81.5° to 147.5°, the Sc–C bond (2.237 A in the transition state) completely breaks apart, and the molecule takes a planar shape. The connections from TS5 to g2-CO2–HScOH and OC(H)OScOH in the forward and reverse directions, respectively, have been confirmed by IRC calculations. The g2-CO2–HScOH ! OC(H)OScOH reaction step is calculated to be 15.4 kcal/mol exothermic and the OC(H)OScOH intermediate lies 51.6 kcal/mol lower in energy than the CO2 + H2 + ScO(2R+) reactants. The OC(H)OScOH intermediate is metastable and can easily rearrange into the cyclic cyc-OC(H)OScOH molecule overcoming a ring closure barrier at TS6. The barrier height is only 2.0 kcal/mol and cycOC(H)OScOH resides 15.4 and 67.0 kcal/mol lower in energy than OC(H)OScOH and CO2 + H2 + ScO(2R+), respectively. The cyc-OC(H)OScOH(2A 0 ) molecule, which includes a ScOCO four-member ring fragment, is Cs-symmetric with the symmetry plane containing H, O, Sc, C, and H atoms, while the other two oxygen atoms are reflected by this plane. This structure represents the most favorable isomer in the CO2/H2/ScO system among all local minima found in this study. The ring closure transition state TS6 is rather early, reactant-like and the changes in the OC(H)OScOH ! cycOC(H)OScOH rearrangement mostly involve the ScOC angle, which decreases from 147.5° in the reactant to 122.2° in TS6 and 88.0° in the product. Otherwise, the ˚ in OC(H)OStwo C–O bond lengths (1.330 and 1.209 A ˚ in TS6) become equal (1.274 cOH and 1.322 and 1.219 A ˚ ) in the cyclic intermediate. A The next reaction step is 1,3-hydrogen transfer from ScOH to an oxygen of the HCO2 fragment. From cycHCO2ScOH to the corresponding transition state TS7, the molecule folds (the dihedral angle between ScOH and ScOC planes becomes 60°) and the breaking O–H bond bends towards one oxygen atom of the HCO2 frag˚ , while the H–O distance ment and lengthens to 1.098 A ˚ . TS7 exhibits a for the forming bond shortens to 1.458 A non-planar six-member ring structure (ScOCOHO). TS7 lies 2.3 kcal/mol above the initial reactants, so that the barrier at this transition state is high, 69.3 kcal/mol relative to cyc-OC(H)OScOH. The reaction product at this stage is another cyclic structure, cyc-HCOO(H)ScO, where the second hydrogen atom is attached to one of the oxygens in the ScOCO four-member ring. cycHCOO(H)ScO lies 12.4 kcal/mol lower in energy than CO2 + H2 + ScO(2R+) and can further undergo a ring opening process leading to the HCO(H)OScO intermediate. The ring opening barrier is relatively low, 11.0 kcal/mol with respect to cyc-HCOO(H)ScO and the
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rearrangement is slightly endothermic, as HCO(H)OScO resides 8.1 kcal/mol below the initial reactants. HCO(H)OScO is a complex of scandium oxide with the trans-conformer of formic acid. The binding energy of the complex is 15.5 kcal/mol and it can decompose to ScO(2R+) + HCOOH without an exit barrier. This step completes the reaction pathway from CO2 + H2 to formic acid in the presence of ScO.
4. Conclusions Summarizing the reaction mechanisms described above, we can conclude that in the gas-phase ScO can catalyze the H2 + CO2 ! HCOOH reaction of hydrogenation of carbon dioxide to formic acid. The most favorable reaction pathway is the following: CO2 + H2 + ScO(2R+) ! H2 + g1-CO2–ScO ! TS1 ! H2 + g2-CO2– ScO ! g2-CO2–ScO–H2 ! TS2 ! g2-CO2–HScOH ! TS5 ! OC(H)OScOH ! TS6 ! cyc-OC(H)OScOH ! TS7 ! cyc-HCOO(H)ScO ! TS8 ! HCO(H)OScO ! trans-HCOOH + ScO(2R+). Thus, ScO first forms an end-on complex with the carbon dioxide molecule, which easily rearranges to a side-on complex and then molecular hydrogen binds to the latter. After that, the ScO fragment inserts into the H–H bond over a low 6.3 kcal/mol barrier and this is followed by the hydrogen migration from Sc to C to produce the OC(H)OScOH intermediate overcoming a barrier of 12.1 kcal/mol. OC(H)OScOH can straightforwardly isomerize into the cyclic cycOC(H)OScOH molecule, the most stable structure in the CO2/H2/ScO system. Then, cyc-OC(H)OScOH can rearrange to cyc-HCOO(H)ScO by the H shift from ScOH to an oxygen atom of HCO2. However, this step is endothermic and a barrier of 69.3 kcal/mol has to be cleared. At the final steps, cyc-HCOO(H)ScO undergoes ring opening to produce the HCO(H)OScO complex and the latter can decompose to ScO(2R+) and the trans-conformer of formic acid thus yielding the product and restoring the catalyst. If the reaction occurs in the gas phase and the energy is slowly dissipated through collisions, the reaction rate mostly depends on the activation barriers (relative energies of transitions states or products if the reaction steps have no barrier) with respect to the initial CO2 + H2 + ScO(2R+) reactants. For these conditions, the activation energy for the ScO-catalyzed hydrogenation of CO2 simply coincides with its endothermicity, 7.4 kcal/mol, otherwise, the highest in energy transition state is TS7, 2.3 kcal/mol above the reactants. In condensed phase, where the collisional deactivation is fast, the more important factors are barriers for individual reaction steps. The highest barrier calculated for the ScO-catalyzed CO2 + H2 ! HCOOH reaction is 69.3 kcal/mol, only few kcal/mol lower than the barrier for the non-catalyzed reaction, 73.8 kcal/mol at the same
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level of theory. This indicates that in condensed phase scandium oxide is not expected to be an efficient catalyst for hydrogenation of carbon dioxide. Our results also shed light on what products can be expected in matrix isolation experiments if the ScO, CO2, and H2 vapors are mixed together. According to the calculated PES, in addition to the bimolecular g2CO2–ScO complex and the HScOH molecule, the trimolecular g2-CO2–ScO–H2 complex, the g1-CO2–HScOH and g2-CO2–HScOH complexes, and the cyc-OC(H)OScOH molecule can be easily produced. On the other hand, if one mixes scandium oxide and formic acid vapors, the intermediates and products, which can be potentially isolated, include the HCO(H)OScO complex and the cyclic cyc-HCOO(H)ScO and cyc-OC(H)OScOH molecules. Acknowledgements Some of the computer equipment was supplied by the National Science Council of Taiwan, R.O.C. A partial support from Tamkang University is also appreciated. A.M.M. is thankful to Florida International University for his start-up funds used to purchase the computer equipment partially employed for this study.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.cplett.2004.07.113.
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Reaction mechanism of hydrogenation of carbon dioxide to formic acid in the presence of scandium oxide: a density functional study Der-Yan Hwang b
a,*
, Alexander M. Mebel
*,b
a Department of Chemistry, Tamkang University, In-tran Rd. 151, Tamsui 25137, Taiwan Department of Chemistry and Biochemistry, Florida International University, Miami, FL 33199, USA
Received 13 July 2004; in final form 13 July 2004
Abstract Density functional calculations of potential energy surfaces in the CO2/H2/ScO(2R+) system have been performed to investigate the reaction mechanism of CO2 hydrogenation to formic acid in the presence of ScO. The results show that ScO can easily form a variety of complexes with CO2 and H2, complexes of CO2 with HScOH, and a highly exothermic cyc-OC(H)OScOH molecule. Although transformation of the latter to ScO + HCOOH is impeded by the high barrier for hydrogen transfer from ScOH to HCO2, it is expected to be fast in the gas phase because the transition state is only 2.3 kcal/mol higher in energy than the reactants CO2 + H2 + ScO. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction Fixation of carbon dioxide has received much attention in recent years [1–6] due to environmental importance of CO2 as a greenhouse gas and its large-scale availability at low cost. CO2 is a highly oxidized and thermodynamically stable compound, which has low reactivity. Therefore, its utilization remains an important problem. Because of the high stability of CO2 and its inertness in chemical reactions, the toxic carbon monoxide instead of CO2 is mostly used currently for many processes in industry as a C1 building unit [6]. On the other hand, a promising approach for CO2 fixation is the use of homogeneous, heterogeneous, or enzymatic catalysis. The homogeneous catalysis for fixation of carbon dioxide has been discussed in recent reviews [2,4]. For instance, catalytic hydrogenation of *
Corresponding author. Fax: +886 2 2620 9924. E-mail address: [email protected] (D.-Y. Hwang).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.07.113
CO2 to formic acid, which provides a prospective technique allowing to use CO2 as a raw material in largescale chemical synthesis, has attracted much interest in recent years. Rhodium and ruthenium phosphine complexes were found to serve as efficient catalysts for the formation of formic acid from CO2 [7–9]. Alternatively, heterogeneous catalysis can provide several technical advantages due to stability, separation, handling and reuse of the catalyst and reactor design. However, at present, the range of compounds that have been synthesized from CO2 by heterogeneous catalytic methods is still relatively narrow and is mainly confined to methanol synthesis, the syntheses of methylamines and formic acid derivatives, and the production of synthesis gas [6]. For example, a ruthenium-containing silica hybrid catalyst has been suggested for the production of various derivatives of formic acid [6]. The CO2 + H2 ! HCOOH reaction is known to have a very high barrier without a catalyst [10–15]. For
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instance, the hydrogenation reaction of carbon dioxide occurs in two steps, H2 addition to CO2 to form cisHCOOH followed by cis–trans isomerization to produce the more stable trans-HCOOH structure. The highest barrier on this pathway is found for the initial H2 addition step and it is calculated to be 76.3 and 73.8 kcal/mol at the G2 [15] and B3LYP/6-311 + G(3df,2p)//B3LYP/ 6-31G(d,p) levels, respectively, and 70.2–73.2 kcal/mol according to experimental measurements [11,12]. In this view, a search of new efficient catalysts for the fixation of carbon dioxide by H2 to produce formic acid remains an active area of research. Metals and their compounds can catalyze many important chemical reactions, either homogeneously or heterogeneously. Transition metal oxides can coordinate CO2 [16–18] and also react with molecular hydrogen [19–21]. Therefore, it would be reasonable to expect that these oxides can assist the conversion of CO2 to formic acid, i.e., catalyze the CO2 + H2 ! HCOOH reaction. In the present work, we use theoretical density functional calculations to investigate the reaction mechanism of carbon dioxide with H2 catalyzed by ScO. Our recent study of the CO/H2/BeO system demonstrated that the addition of molecular hydrogen to carbon monoxide can be significantly enhanced in the presence of BeO [22]. In a similar way, we can expect that the presence of a metal oxide, ScO in this case, can facilitate the addition of H2 to CO2 producing formic acid. Scandium oxide reacts with molecular hydrogen producing HScOH [19,23]. The latter may then react with CO2 and the metal-oxide-bonded H atoms of HScOH can be eventually transferred to the carbon dioxide fragment. Experimentally, ScO has been thoroughly studied in the gas phase and matrix probably due to its appearance in the spectra of stars (see, for example [24–27]). As the lightest among transition metals, Sc is also very convenient for ab initio calculations, and a number of theoretical works have been devoted to ScO and its reactions [19,23,28–31]. However, the interaction mechanism of scandium oxide with the CO2/H2 mixture was not known so far and in this Letter we consider the CO2 + H2 + ScO reaction. We report potential energy surface (PES) for this reaction, elucidate the gas-phase reaction mechanism and provide geometric structures of various reactants, products, intermediates, and transition states as well as their reliable energies. This work continues our systematic studies of reaction mechanisms for various chemical reactions, including conversion of methane to methanol [32], nitrogen hydrogenation [33,34], and conversion of CO to formaldehyde [22], in the presence of metal oxides. A detailed understanding of elementary reaction steps from reactants to desired product and a comparison of the catalytic role of different transition metal oxides would be helpful to facilitate a rational design of potential catalysts.
2. Computational details Full geometry optimizations were carried out at the B3LYP/6-31G(d,p) level of theory to locate various stationary points (reactants, intermediates, transition states, and products) on the ground doublet electronic state PES of the CO2/H2/ScO system. Harmonic vibrational frequencies were calculated at the same B3LYP/6-31G(d,p) level in order to characterize the stationary points as minima or transition states, to obtain zero-point vibrational energy corrections (ZPE), and to generate force constants needed for intrinsic reaction coordinate (IRC) calculations, which were employed to confirm connections of transition states to certain local minima. The relative energies were refined using single-point calculations with B3LYP/6-31G(d,p) optimized geometry employing the B3LYP method with the larger 6-311 + G(3df,2p) basis set. All density functional calculations described here were performed employing the GAUSSIAN 98 program [35].
3. Results and discussion Here we consider the lowest in energy doublet electronic state for the CO2 + H2 + ScO(2R+) reaction. The total energies, ZPE, and relative energies of various reactants, products, intermediates, and transition states for this reaction are presented in Table 1, their vibrational frequencies are shown in Table 2. The potential energy diagram along the CO2 + H2 + ScO(2R+) ! HCOOH + ScO(2R+) reaction pathway is depicted in Fig. 1 and optimized geometries of the intermediates and transition states are illustrated in Fig. 2. 3.1. Formation of CO2–ScO–H2 and CO2–HScOH complexes There are two different possibilities for initiation of the CO2 + H2 + ScO(2R+) reaction. Scandium oxide can react first either with H2 or with carbon dioxide. We first consider the coordination of carbon dioxide to ScO. The two can form a relatively weak end-on g1-CO2–ScO complex without a barrier. The binding energy of the complex is calculated to be 3.6 kcal/mol. The geometry of the CO2 fragment in the complex changes only slightly; the coordinated C–O bond elongates by ˚ as compared to that in isolated CO2 and the 0.02 A O–C–O angle is close to 180°. The only notable change in vibrational frequencies in g1-CO2–ScO as compared to isolated CO2 and ScO is a decrease of the degenerate O–C–O bending frequency from 666 cm1 in carbon dioxide to 561 and 564 cm1 in the complex (see Table 2). The interaction between two molecular fragments in g1-CO2–ScO is described in terms of donation of electron density from the occupied pp orbital of CO2 to
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Table 1 Total energies (hartree), ZPE, and relative energies (kcal/mol) of various species in the CO2 + H2 + ScO reaction calculated at the B3LYP/6-31G(d,p) and B3LYP/6-311 + G(3df,2p)//B3LYP/6-31G(d,p) levels of theory Species
H2 ScO(2R+) CO2 CO2 + ScO(2R+) H2 + ScO(2R+) CO2 + H2 + ScO(2R+) g1-CO2–ScO(2A 0 ) TS1 g2-CO2–ScO g2-CO2–ScO + H2 g2-CO2–ScO–H2 TS2 g2-CO2–HScOH TS3(2A 0 ) HScOH(2A 0 ) CO2 + HScOH(2A 0 ) g1-CO2–HScOH TS4 TS5 OC(H)OScOH(2A 0 ) TS6(2A 0 ) cyc-OC(H)OScOH(2A 0 ) TS7 cyc-HCOO(H)ScO TS8 HCO(H)OScO HCOOH HCOOH + ScO(2R+)
B3LYP/6-31G**
B3LYP/6-311 + G(3df,2p)
Total energy
ZPE
1.17854 835.89460 188.58094 1024.47554 837.07314 1025.65408 1024.48820 1024.48638 1024.49827 1025.67681 1025.68510 1025.67470 1025.72165 837.05518 837.09746 1025.67840 1025.71100 1025.69222 1025.69884 1025.75111 1025.75031 1025.77566 1025.66796 1025.69338 1025.67791 1025.68441 189.76222 1025.65682
6.39 1.50 7.28 8.77 7.88 15.16 9.02 8.03 8.55 14.93 17.84 16.91 18.01 9.51 10.36 17.57 17.45 17.57 18.20 22.23 22.15 22.51 20.02 22.21 22.28 22.72 21.33 22.83
Relative energy
0 0 0 7.69 7.55 14.49 14.49 16.78 11.19 39.55 12.90 12.78 12.78 33.42 21.52 25.04 53.82 53.39 68.94 3.85 17.60 7.83 11.47 5.95
Total energy 1.18001 835.96909 188.65985 1024.62894 837.14910 1025.80895 1024.63505 1024.63217 1024.64479 1025.82480 1025.83345 1025.82189 1025.87123 837.12984 837.17561 1025.83546 1025.86154 1025.84674 1025.85226 1025.90242 1025.89920 1025.92742 1025.81308 1025.83994 1025.82252 1025.83395 189.84022 1025.80931
Relative energy
0 0 0 3.58 2.77 10.17 10.17 12.69 6.37 36.23 13.69 14.18 14.18 30.71 21.30 24.14 51.58 49.64 66.99 2.27 12.39 1.40 8.13 7.45
Table 2 Vibrational frequencies (cm1) of various compounds in the CO2 + H2 + ScO reaction calculated at the B3LYP/6-31G(d,p) level Species 1
g -CO2–ScO TS1 g2-CO2–ScO g2-CO2–ScO–H2 TS2 g2-CO2–HScOH TS3 HScOH g1-CO2–HScOH TS4 TS5 OC(H)OScOH TS6 cyc-OC(H)OScOH TS7 cyc-HCOO(H)ScO TS8 HCO(H)OScO
Frequencies 26, 37, 103, 178, 561, 564, 1032, 1370, 2441 280i, 25, 103, 131, 366, 395, 1031, 1238, 2324 70, 153, 197, 348, 404, 689, 1020, 1175, 1922 40, 73, 145, 198, 211, 349, 398, 484, 678, 698, 981, 1011, 1184, 1915, 4115 1191i, 73, 98, 163, 204, 358, 418, 703, 886, 952, 1180, 1271, 1703, 1899, 1918 74, 129, 179, 221, 231, 358, 373, 405, 507, 711, 795, 1173, 1563, 1922, 3964 1364i, 885, 949, 1256, 1649, 1911 302, 327, 342, 764, 1523, 3944 45, 70, 101, 143, 162, 248, 384, 395, 528, 742, 803, 1191, 1570, 1864, 3959 54i, 33, 105, 156, 183, 279, 295, 371, 477, 514, 735, 1316, 1468, 2412, 3949 764i, 78, 130, 294, 315, 386, 391, 462, 692, 732, 758, 1093, 1506, 1928, 3962 34, 47, 121, 139, 365, 368, 504, 743, 775, 1054, 1240, 1413, 1801, 2992, 3949 90i, 30, 84, 170, 375, 388, 519, 743, 774, 1060, 1251, 1403, 1757, 3009, 3939 38, 60, 235, 326, 358, 376, 379, 739, 818, 1060, 1351, 1389, 1591, 3057, 3969 998i, 141, 177, 256, 297, 569, 674, 829, 938, 1090, 1255, 1357, 1389, 1913, 3134 105, 146, 183, 309, 405, 460, 616, 721, 978, 1002, 1089, 1263, 1411, 3009, 3841 111i, 59, 128, 213, 236, 423, 672, 731, 1021, 1057, 1222, 1351, 1635, 3117, 3732 66, 112, 166, 279, 446, 466, 587, 850, 997, 1061, 1271, 1289, 1462, 3084, 3764
empty d orbitals of Sc and back donation from the dr MO of ScO to the empty antibonding pr* MO of CO2 and, judging from the binding energy of g1-CO2–ScO, this interaction is rather weak. However, it is stronger than that between N2 and ScO, for which no end-on
complex was found on PES [34]. g1-CO2–ScO represents only a metastable local minimum on PES and easily rearranges to the side-on g2-CO2–ScO complex via a low barrier of 0.8 kcal/mol at TS1. The side-on complex is much more strongly bound than g1-CO2–ScO and lies
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CO2+TS3 13.7 0.0 CO2+H2+ScO(2Σ+)
η1-CO2-ScO+H2
TS1 -2.8
-3.6
TS2 -6.4
η2-CO2-ScO+H2 -12.7 -10.2
-14.2
η2-CO2-ScO-H2
CO2+HScOH
TS4 -21.3
-30.7 1
η -CO2-HScOH
-36.2 η2-CO2-HScOH
(a)
TS7 2.3
TS8 -1.4 -12.4
HCOOH+ScO(2Σ+) 7.4 -8.1 HCO(H)OScO
cyc-HCOO(H)ScO TS5 -24.1 -36.2 2
η -CO2-HScOH -51.6
TS6 -49.6
OC(H)OScOH
(b)
-67.0 cyc-OC(H)OScOH
Fig. 1. Potential energy diagram for the CO2 + H2 + ScO(2R+) ! HCOOH + ScO(2R+) reaction calculated at the B3LYP/6-311 + G(3df,2p)// B3LYP/6-31G(d,p) + ZPE[B3LYP/6-31G(d,p)] level: (a) formation of various CO2/ScO/H2 and CO2/HScOH complexes; (b) pathway leading to formic acid. Relative energies of various species are given in kcal/mol.
10.2 kcal/mol lower in energy than CO2 + ScO. The distortion of the CO2 fragment in g2-CO2–ScO is also much more significant; the coordinated C–O bond is ˚ , the other C–O bond is also 0.03 lengthened by 0.11 A ˚ longer than that in free carbon dioxide, and the A O–C–O angle decreases to 141.3°. These geometric changes are reflected in vibrational frequencies of g2CO2–ScO. For instance, the C–O asymmetric stretch frequency decreases from 2438 cm1 in CO2 to 1922 cm1 in the complex, the C–O symmetric stretch frequency changes from 1376 to 1175 cm1, and the O–C–O bending frequencies become 404 and 689 cm1. These changes are large enough to be observable in FTIR matrix isolation experiments if ScO is mixed with carbon dioxide, especially for the IR-intense C–O asymmetric stretch and the O–C–O bending vibrations. The symmetric stretch vibration, which is not IR-active in free CO2, also may become observable in the complex. The Sc–O stretching frequency is less sensitive to the complex formation; it changes only slightly from 1048 cm1 in ScO to 1020 cm1 in g2-CO2–ScO, as calculated
at the same B3LYP/6-31G(d) level of theory. The interaction between the fragments in the g2-CO2–ScO complex is similar to that in g2-N2–ScO considered earlier [34]: donation of electron density from the bonding pp MO of CO2 to the empty dr orbital of Sc accompanied with back donation from the singly occupied dd Sc orbital to the antibonding pp* MO of CO2. To make such interaction possible, the single electron from the dr orbital of scandium atom in the ground electronic state of ScO has to be promoted to the dd orbital; this causes the appearance of the barrier at TS1 between the g1- and g2-CO2–ScO complexes. g2-CO2–ScO can in turn form another, weakly bound trimolecular complex with H2, g2-CO2–ScO–H2, 2.5 and 12.7 kcal/mol lower in energy than the bimolecular sideon complex of CO2 with ScO and the initial reactants, respectively. In the complex, the H2 and CO2 ligands attached to the Sc atom lie in perpendicular planes and H2 interacts with ScO through the rg ! dr (Sc) donation. The Sc atom in g2-CO2–ScO is better prepared for this kind of interaction with molecular hydrogen as its single
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Fig. 2. Geometries of various intermediates and transition states of the CO2 + H2 + ScO(2R+) ! HCOOH + ScO(2R+) reaction optimized at the B3LYP/6-31G(d,p) level. Bond lengths are given in angstroms and bond angles in degrees.
electron is already promoted to the dd-type orbital and the dr orbital is empty. No molecular complex exists between H2 and the ground state ScO(2R+), such complex was found only for the excited 2D state of scandium oxide [34], which is formed by the dr ! dd electronic
transition. Due to coordination to the scandium atom, ˚ and the the bond length in H2 increases by 0.02 A H–H vibrational frequency changes from 4419 to 4115 cm1. However, this vibration is IR-inactive in free H2 and remains very weak in the complex. The changes of
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the C–O and Sc–O stretching and O–C–O bending frequencies from g2-CO2–ScO to g2-CO2–ScO–H2 are minor, within 10 cm1, but the vibrational spectrum of the latter is calculated to have additional peaks at 40, 211, 484, 678, and 981 cm1 (see Table 2). The g2CO2–ScO–H2 complex isomerizes to g2-CO2–HScOH by insertion of ScO into the H-H bond via transition state TS2. The barrier for this process is 6.3 kcal/mol relative to g2-CO2–ScO–H2 and TS2 lies 6.4 kcal/mol lower in energy than CO2 + H2 + ScO(2R+). The g2-CO2– HScOH intermediate is 36.2 kcal/mol more stable than the reactants. The structure and orientation of the CO2 fragment in g2-CO2–HScOH are similar to those in g2-CO2–ScO; the coordinated C@O bond is stretched ˚ as compared to the bond length in isolated by 0.12 A CO2, the second C@O bond length increases by 0.03 ˚ , and the OCO angle is 140.8°. Similar to g2-CO2– A ScO, the bonding between the fragments in g2-CO2– HScOH can be described in terms of electron donation from the bonding pp MO of CO2 to the empty dr-type orbital of Sc and back donation from a singly occupied dd-type orbital of the metal atom to the antibonding pp* MO of CO2. Earlier [34], we found a similar side-on complex of HScOH with N2, but g2-N2–HScOH is less strongly bound with respect to N2 + HScOH, by 16.8 kcal/mol as compared to 22.0 kcal/mol for g2-CO2– HScOH. Now we consider the alternative pathway leading from CO2 + H2 + ScO(2R+) to g2-CO2–HScOH starting from the studied earlier ScO + H2 reaction [19]. In the ground electronic state, scandium oxide can insert into the H–H bond via a Cs-symmetric transition state TS3(2A 0 ) with a barrier of 13.7 kcal/mol. After the insertion, the planar HScOH (3A 0 ) molecule is formed, which lies 14.2 kcal/mol lower in energy that the initial reactants. From a comparison between TS3 and TS2 (insertion transition states of isolated ScO(2R+) and the ScO fragment of g2-CO2–ScO, respectively, into H2), we can see that the latter has an earlier character than the former displayed in the shorter H–H distance for the breaking bond and longer O–H and Sc–H lengths for the forming bonds. The barrier at TS2 (with respect to the reactant at this particular reaction step) is 7.4 kcal/ mol lower than that at TS3 indicating that the insertion process becomes more facile due to the coordination of CO2 to ScO. This can be attributed to the fact that in the g2-CO2–ScO complex the single electron of Sc is promoted from the dr to dd orbital and therefore the donation of electron density from rg(H2) to dr(Sc) and back donation from dd(Sc) to ru*(H2) taking place during the insertion process occur more easily. At the next reaction step, HScOH can form a strong end-on complex with carbon dioxide, g1-CO2–HScOH, bound by 16.5 kcal/ mol relative to the separated HScOH and CO2 molecules. The complex formation occurs without a barrier and g1-CO2–HScOH lies 30.7 kcal/mol lower in energy
than CO2 + H2 + ScO(2R+). In HScOH, the unpaired electron still occupies the dr orbital of Sc and the mechanism of complex formation is similar to that for g1CO2–ScO, electron donation from the pp orbital of CO2 to empty d orbitals of Sc and back donation from the dr MO of ScO to the pr* MO of CO2. Interestingly, the end-on complex of CO2 with HScOH is stronger than with ScO(2R+); thus, the presence of one ligand, H2, increases the binding energy with the second ligand, CO2, and vice versa. g1-CO2–HScOH can rearrange to the side-on g2-CO2–HScOH complex over a 9.4 kcal/ mol barrier at TS4. As in the case of the CO2–ScO complexes, the barrier originates from the necessity to promote the single electron from the dr to dd orbital on Sc. The side-on complex is 5.5 kcal/mol more favorable than g1-CO2–HScOH. Comparing vibrational spectra of isolated HScOH + CO2 and g1-CO2–HScOH and g2-CO2–HScOH complexes, we can see a blue shift for the frequencies of HScOH (O–H stretch: from 3944 cm1 in HScOH to 3959 and 3964 cm1 in g1 and g2, respectively; Sc–H stretch: from 1523 cm1 to 1570 and 1563 cm1; and Sc–O stretch: from 764 cm1 to 803 and 795 cm1) and a red shift for the CO2 frequencies (asymmetric C–O stretch: from 2438 cm1 in CO2 to 1864 and 1922 cm1 in g1 and g2, respectively; symmetric C–O stretch: from 1376 cm1 to 1191 and 1173 cm1). A comparison of the two mechanisms for the formation of g2-CO2–HScOH shows that the ÔCO2 first, then H2Õ pathway, CO2 + H2 + ScO(2R+) ! H2 + g1-CO2– ScO ! TS1 ! H2 + g2-CO2–ScO ! g2-CO2–ScO–H2 ! TS2 ! g2-CO2–HScOH, is clearly preferable as compared to the ÔH2 first, then CO2Õ pathway, CO2 + H2 + ScO(2R+) ! TS3 ! CO2 + HScOH ! g1CO2–HScOH ! TS4 ! g2-CO2–HScOH, because for the latter the barrier of 13.7 kcal/mol relative to the initial reactants has to be overcome. For the former, owing to the barrier decrease for the insertion into the H–H bond and to the fact that the g2-CO2–ScO–H2 complex is significantly bound with respect to the initial reactants, the transition state TS2 lies 6.4 kcal/mol below CO2 + H2 + ScO(2R+). Therefore, the CO2 + H2 + ScO(2R+) reaction through the ÔCO2 first, then H2Õ mechanism is expected to be fast (all intermediates and transition states are lower in energy than the reactants) and the g2-CO2–HScOH complex can be readily produced. 3.2. Formation of formic acid The g2-CO2–HScOH complex can isomerize to the planar OC(H)OScOH intermediate by 1,3-hydrogen migration from Sc to the C atom via TS5. The barrier for this process is calculated to be 12.1 kcal/mol and the transition state resides 24.1 kcal/mol lower in energy than the initial reactants. TS5 has a four-member ring
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ScHCO fragment and an early character with the break˚ longer than that in the g2-CO2– ing Sc–H bond 0.04 A ˚ HScOH reactant and the forming C–H bond 0.63 A longer than that in the reaction product. After the barrier at TS5 is cleared, the COSc angle eventually in˚ creases from 81.5° to 147.5°, the Sc–C bond (2.237 A in the transition state) completely breaks apart, and the molecule takes a planar shape. The connections from TS5 to g2-CO2–HScOH and OC(H)OScOH in the forward and reverse directions, respectively, have been confirmed by IRC calculations. The g2-CO2–HScOH ! OC(H)OScOH reaction step is calculated to be 15.4 kcal/mol exothermic and the OC(H)OScOH intermediate lies 51.6 kcal/mol lower in energy than the CO2 + H2 + ScO(2R+) reactants. The OC(H)OScOH intermediate is metastable and can easily rearrange into the cyclic cyc-OC(H)OScOH molecule overcoming a ring closure barrier at TS6. The barrier height is only 2.0 kcal/mol and cycOC(H)OScOH resides 15.4 and 67.0 kcal/mol lower in energy than OC(H)OScOH and CO2 + H2 + ScO(2R+), respectively. The cyc-OC(H)OScOH(2A 0 ) molecule, which includes a ScOCO four-member ring fragment, is Cs-symmetric with the symmetry plane containing H, O, Sc, C, and H atoms, while the other two oxygen atoms are reflected by this plane. This structure represents the most favorable isomer in the CO2/H2/ScO system among all local minima found in this study. The ring closure transition state TS6 is rather early, reactant-like and the changes in the OC(H)OScOH ! cycOC(H)OScOH rearrangement mostly involve the ScOC angle, which decreases from 147.5° in the reactant to 122.2° in TS6 and 88.0° in the product. Otherwise, the ˚ in OC(H)OStwo C–O bond lengths (1.330 and 1.209 A ˚ in TS6) become equal (1.274 cOH and 1.322 and 1.219 A ˚ ) in the cyclic intermediate. A The next reaction step is 1,3-hydrogen transfer from ScOH to an oxygen of the HCO2 fragment. From cycHCO2ScOH to the corresponding transition state TS7, the molecule folds (the dihedral angle between ScOH and ScOC planes becomes 60°) and the breaking O–H bond bends towards one oxygen atom of the HCO2 frag˚ , while the H–O distance ment and lengthens to 1.098 A ˚ . TS7 exhibits a for the forming bond shortens to 1.458 A non-planar six-member ring structure (ScOCOHO). TS7 lies 2.3 kcal/mol above the initial reactants, so that the barrier at this transition state is high, 69.3 kcal/mol relative to cyc-OC(H)OScOH. The reaction product at this stage is another cyclic structure, cyc-HCOO(H)ScO, where the second hydrogen atom is attached to one of the oxygens in the ScOCO four-member ring. cycHCOO(H)ScO lies 12.4 kcal/mol lower in energy than CO2 + H2 + ScO(2R+) and can further undergo a ring opening process leading to the HCO(H)OScO intermediate. The ring opening barrier is relatively low, 11.0 kcal/mol with respect to cyc-HCOO(H)ScO and the
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rearrangement is slightly endothermic, as HCO(H)OScO resides 8.1 kcal/mol below the initial reactants. HCO(H)OScO is a complex of scandium oxide with the trans-conformer of formic acid. The binding energy of the complex is 15.5 kcal/mol and it can decompose to ScO(2R+) + HCOOH without an exit barrier. This step completes the reaction pathway from CO2 + H2 to formic acid in the presence of ScO.
4. Conclusions Summarizing the reaction mechanisms described above, we can conclude that in the gas-phase ScO can catalyze the H2 + CO2 ! HCOOH reaction of hydrogenation of carbon dioxide to formic acid. The most favorable reaction pathway is the following: CO2 + H2 + ScO(2R+) ! H2 + g1-CO2–ScO ! TS1 ! H2 + g2-CO2– ScO ! g2-CO2–ScO–H2 ! TS2 ! g2-CO2–HScOH ! TS5 ! OC(H)OScOH ! TS6 ! cyc-OC(H)OScOH ! TS7 ! cyc-HCOO(H)ScO ! TS8 ! HCO(H)OScO ! trans-HCOOH + ScO(2R+). Thus, ScO first forms an end-on complex with the carbon dioxide molecule, which easily rearranges to a side-on complex and then molecular hydrogen binds to the latter. After that, the ScO fragment inserts into the H–H bond over a low 6.3 kcal/mol barrier and this is followed by the hydrogen migration from Sc to C to produce the OC(H)OScOH intermediate overcoming a barrier of 12.1 kcal/mol. OC(H)OScOH can straightforwardly isomerize into the cyclic cycOC(H)OScOH molecule, the most stable structure in the CO2/H2/ScO system. Then, cyc-OC(H)OScOH can rearrange to cyc-HCOO(H)ScO by the H shift from ScOH to an oxygen atom of HCO2. However, this step is endothermic and a barrier of 69.3 kcal/mol has to be cleared. At the final steps, cyc-HCOO(H)ScO undergoes ring opening to produce the HCO(H)OScO complex and the latter can decompose to ScO(2R+) and the trans-conformer of formic acid thus yielding the product and restoring the catalyst. If the reaction occurs in the gas phase and the energy is slowly dissipated through collisions, the reaction rate mostly depends on the activation barriers (relative energies of transitions states or products if the reaction steps have no barrier) with respect to the initial CO2 + H2 + ScO(2R+) reactants. For these conditions, the activation energy for the ScO-catalyzed hydrogenation of CO2 simply coincides with its endothermicity, 7.4 kcal/mol, otherwise, the highest in energy transition state is TS7, 2.3 kcal/mol above the reactants. In condensed phase, where the collisional deactivation is fast, the more important factors are barriers for individual reaction steps. The highest barrier calculated for the ScO-catalyzed CO2 + H2 ! HCOOH reaction is 69.3 kcal/mol, only few kcal/mol lower than the barrier for the non-catalyzed reaction, 73.8 kcal/mol at the same
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level of theory. This indicates that in condensed phase scandium oxide is not expected to be an efficient catalyst for hydrogenation of carbon dioxide. Our results also shed light on what products can be expected in matrix isolation experiments if the ScO, CO2, and H2 vapors are mixed together. According to the calculated PES, in addition to the bimolecular g2CO2–ScO complex and the HScOH molecule, the trimolecular g2-CO2–ScO–H2 complex, the g1-CO2–HScOH and g2-CO2–HScOH complexes, and the cyc-OC(H)OScOH molecule can be easily produced. On the other hand, if one mixes scandium oxide and formic acid vapors, the intermediates and products, which can be potentially isolated, include the HCO(H)OScO complex and the cyclic cyc-HCOO(H)ScO and cyc-OC(H)OScOH molecules. Acknowledgements Some of the computer equipment was supplied by the National Science Council of Taiwan, R.O.C. A partial support from Tamkang University is also appreciated. A.M.M. is thankful to Florida International University for his start-up funds used to purchase the computer equipment partially employed for this study.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.cplett.2004.07.113.
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