Article pubs.acs.org/JPCC
Mechanism of 2‑Ethylhexenal Hydrogenation on Pd(111): A Density Functional Study Jingjing Ji,†,‡ Liang Zhao,*,†,‡ Dan Wang,§ Jinsen Gao,‡ Chunming Xu,‡ and Huangfan Ye‡ ‡
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, 102249, People’s Republic of China Daqing Petrochemical Research Center, Petrochemical Research Institute of PetroChina, Daqing 163714, People’s Republic of China
§
ABSTRACT: The detailed hydrogenation processes of 2-ethylhexenal on Pd(111) toward 2-ethylhexanol were investigated by density functional theory (DFT) calculations to understand the hydrogenation mechanism of 2-ethylhexenal. Several adsorption modes of 2-ethylhexenal on Pd(111) were studied. The adsorption of cis-conformers on the Pd(111) surface was found to be more stable than the trans-conformers; however, cisisomers are less stable in the gas phase. Both E-η3-trans and E-η4-trans modes were used to probe the hydrogenation of 2-ethylhexenal, although the former plays a primary role in hydrogenation reactions. Several plausible reaction pathways were calculated. For Eη3-trans mode, C3 → C2 → O → C1, C3 → C2 → C1 → O, C2 → C3 → O → C1, and C2 → C3 → C1 → O routes are feasible to produce saturated alcohol. However, for the E-η4-trans mode, the formation of the 2-ethylhexanal intermediate, which is a saturated aldehyde, appeared to be easy through both C2 → C3 and C3 → C2 pathways because of low active barriers. Only 2-ethylhexanal generated via the C3 → C2 route was presumed to be available to generate 2-ethylhexanol on E-η4-trans mode. In general, the consecutive hydrogenation reaction of 2-ethylhexenal to 2-ethylhexanal and then to 2-ethylhexanol on the Pd(111) surface determines the whole reaction process and even becomes the rate-limiting step.
1. INTRODUCTION 2-Ethylhexanol is a very significant raw material that has been extensively used in the production of adhesives, plasticizers, surfactants, antioxidants, cosmetics, and additives of diesel and lubricating oil.1−3 It can also be upgraded via esterification to produce a diesel fuel with a high cetane number.4,5 2Ethylhexanol is generally obtained by the aldol condensation of n-butanal and subsequent hydrogenation of the resulting 2ethylhexenal. Hydrogenation of 2-ethylhexenal is considerably significant because it directly affects the quality and yield of 2ethylhexanol. Hydrogenation is a typical reaction of α,βunsaturated aldehydes. Considering the adjacent conjugated CC and CO double bonds in these aldehydes, totally different products may be produced from the same aldehyde depending on the selectivity of catalysts to form saturated aldehydes by CC bond hydrogenation only, to form unsaturated alcohols by CO bond hydrogenation only, or to form saturated alcohols by the hydrogenation of both CC and CO double bonds.1,6−8 Therefore, based on the target products, suitable hydrogenation catalysts, in particular, active metals, are necessary for highly selective hydrogenation. Many experimental studies have been conducted to investigate the catalytic performance of different metals on the hydrogenation of 2-ethylhexenal, among which nickel-based catalysts are predominantly applied to produce 2-ethylhexanol. 9−13 However, the quality of the product is substantially affected by the presence of some intermediates, such as 2-ethylhexanal and 2-ethylhexenol, as well as residual reactant (2-ethylhexenal), because of the low selectivity and low activity of nickel catalysts.14,15 Studies on the consecutive © XXXX American Chemical Society
hydrogenation of 2-ethylhexenal to 2-ethylhexanal then to 2ethylhexanol on Pd is extremely selective compared with that on Ni.16−19 To regulate the different possible reactions and gain the desired product, a good understanding of the selectivity of the 2-ethylhexanal intermediate is beneficial. Based on experiments, the extreme selectivity of Pd for 2-ethylhexanal is mainly related to the adsorption ability of the saturated aldehyde on Pd. Weak adsorption of saturated carbonyl compounds on Pd catalysts has been proposed indirectly by examination of liquid-phase hydrogenation kinetics.20 Although conclusions from adsorption measurements, which could provide the first direct experimental evidence, showed that the adsorbed amount of 2-ethylhexanal is significantly lower when it is adsorbed simultaneously with 2-ethylhexenal, responses are remarkably similar from both compounds when they are adsorbed separately.17 Nevertheless, a detailed conclusion is difficult to arrive at because of the absence of knowledge on the catalytic mechanism of 2-ethylhexenal on the Pd surface. Density functional theory (DFT) is a useful approach in investigating reaction mechanisms. With regard to DFT calculations on α,β-unsaturated aldehydes, the adsorption of acrolein on Pt(111)21−24 and other metal surfaces25−27 has received extensive attention. Adsorption energies and several stable configurations were confirmed by DFT calculations, which offer some insights for the study of 2-ethylhexenal Received: August 26, 2014 Revised: January 8, 2015
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The Journal of Physical Chemistry C adsorption on Pd. Studies on the hydrogenation process of α,βunsaturated aldehydes are scarce. Li et al.28 studied the hydrogenation of acrolein without catalysts by DFT and found that the two pathways for CC hydrogenation have different energy barriers at 75.44 kcal/mol for TSC1 and 87.68 kcal/mol for TSC2. Hydrogenation reactions of acrolein, crotonaldehyde, and prenal on Pt(111) were also calculated in detail, which showed that active barriers for CC and C O hydrogenation are above 18.45 kcal/mol and within 4.61− 18.45 kcal/mol, respectively.29 However, studies on the mechanism of 2-ethylhexenal hydrogenation on metals have not been reported. To obtain better knowledge on the selectivity of the Pd surface to 2-ethylhexanal and to investigate the consecutive hydrogenation of 2-ethylhexenal, quantum-chemical calculations based on DFT were conducted in this paper. In the present study, the adsorption energies and configurations of 2ethylhexenal on Pd surface were calculated first, followed by the design of several possible reaction pathways and then comparison of active barriers among different routes. This study aims to elucidate and provide a deep understanding on the mechanism of 2-ethylhexenal hydrogenation on Pd.
Pd−Pd bond angle is 60° in their bulk geometry, in quite good agreement with 2.75 Å and 60° reported.40,41 With respect to the adsorption modes of α,β-unsaturated aldehydes on metals, Delbecq and Sautet21,22 proposed several adsorption modes of acrolein on Pt(111) and discussed the adsorption stability of acrolein under different conditions, in which both configurational isomers (cis and trans) of acrolein were considered. In their studies, adsorption geometries can be classified into four classes, namely, interaction by the CC or CO bond (with possible di-σ or π structure to two or one metal atom, respectively), simultaneous interaction by both bonds (η4), and interaction by the oxygen lone pair. The η4trans mode is found to be most stable with the lowest coverage of 1/12, η3-cis is most stable at increased coverages θ of 1/9 and 1/6, and di-σCC is most stable at high coverages of 1/4, which could be explained by the steric interactions caused by exposure to different coverages. Moreover, further investigations on the adsorption of acrolein on Pt(111) at various coverages were performed.23,42 The substitution of the C2C3 bond of acrolein by ethyl and propyl groups results in relatively larger molecules, such as 2-ethylhexenal. Nevertheless, studies on the configurational isomers of 2-ethylhexenal have not been reported. According to initial calculations, both Z- and Eisomers of 2-ethylhexenal molecule are theoretically possible. For Z-isomer of 2-ethylhexenal, cis and trans conformers are exhibited in Figure 1a and b separately, and for E-isomer, cis
2. COMPUTATIONAL DETAILS Calculations are based on the density functional theory (DFT) using the generalized gradient approximation (GGA)30 with the exchange-correlation functional developed by the Perdew, Burke, and Ernzerhof31 with Grimme’s dispersion correction, PBE-D2,32 as implemented in the Dmol3 package.33−35 The employed localized double-numerical basis set with polarization functions (DNP) were comparable in size to the Gaussian basis sets 6-31G** but proven to be more accurate.36,37 The orbital cutoff is 4.5 Å. We used density functional semicore pseudopotentials (DSPPs) for metals. The pseudopotentials are intended for use with density functional local orbital methods, such as the Dmol3 method.38 In adsorption calculations, a 3 × 3 × 1 Monkhorst−Pack k-point grid was used. Structural optimizations were obtained on the basis of the convergence criterion that SCF tolerance was 1.0 × 10−6 Ha, and convergence tolerances of energy, maximum force, and maximum displacement applied during geometry optimization were 1.0 × 10−5 Ha, 2.0 × 10−3 Ha/Å, and 5.0 × 10−3 Å, respectively. Smearing was employed to accelerate convergence of orbital occupation with convergence value of 2.0 × 10−3 Ha. Transition state (TS) searches were carried out at the same level with the Linear Synchronous Transit (LST)/Quadratic Synchronous Transit (QST) methods.39 In this method, the energy maximum along the LST pathway was searched using the reactant and product energy points, followed by a conjugate gradient optimization of this configuration that was used to perform QST maximization. After that, another conjugated gradient minimization was performed. Such a cycle would proceed until the calculation was converged. Four-layer slab model of 5 × 5 atoms, a large box (14 × 14 × 22 Å3), is used to calculate adsorption of one 2-ethylhexenal molecule on Pd(111), which gives a coverage (θ) of 1/25. The bottom two layer atoms are fixed at the positions, and the uppermost two layers are relaxed in the calculations. The vacuum thickness on the top layer is set as 15 Å to separate the surface from its periodic image in the direction along the surface normal. After optimization based on the parameters set, the Pd−Pd bond length is calculated as 2.751 Å, and the Pd−
Figure 1. Optimized molecular structures of 2-ethylhexenal (distances in Å, angles in degrees). Several carbon atoms are marked as 1−6: (a) Z-cis-2-ethylhexenal, (b) Z-trans-2-ethylhexenal, (c) E-cis-2-ethylhexenal, and (d) E-trans-2-ethylhexenal.
and trans conformers are shown in Figure 1c and d, respectively. As for each isomer, two conformers (cis and trans) can be obtained by rotation around the C2−C3 bond, and both CO and CC groups of 2-ethylhexenal are always in the same plane after optimization, as shown in Figure 1. Based on fundamental computations, Z-trans conformation is 2.10 kcal/mol more stable than Z-cis, E-trans conformation is 2.41 kcal/mol more stable than E-cis, and E-trans, which is the most stable in the gas phase, is 2.19 kcal/mol more stable than Z-trans. Considering the low coverage θ of 1/25, η4 and η3 configurations with both cis and trans conformers of E- and Zisomers were chosen to study the adsorption of 2-ethylhexenal on the Pd surface. Figure 2 shows the adsorption configurations of E-isomers on Pd(111), where the a and b lattice vectors coincide with the [11̅0] and [101̅] directions of the Pd crystal, respectively. On η3-cis and η3-trans (Figure 2a and b separately), both double bonds are parallel to Pd surface, and hence, both carbon atoms in the CC interact with Pd atoms separately, B
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Figure 2. Lateral views of several adsorption modes of E-2-ethylhexenal ((a) E-η3-cis, (b) E-η3-trans, (c) E-η4-cis, and (d) E-η4-trans; Adsorption modes of Z-isomers are not shown here). The red ball means the oxygen atom, and the green balls represent the first layer palladium atoms, and the black dashed lines describe the interaction between 2-ethylhexenal molecule and metal surface.
Table 1. Adsorption Energies (in kcal/mol) and Important Geometrical Parameters on the Adsorption of 2-Ethylhexenal Structures on Pd(111) (in Å) Z-η3-cis Z-η3-trans Z-η4-cis Z-η4-trans E-η3-cis E-η3-trans E-η4-cis E-η4-trans
Eads
PdC1
PdC2
PdC3
PdO
C1C2
C2C3
C1O
−74.8 −64.3 −74.2 −59.2 −74.4 −71.7 −73.3 −59.5
2.374 2.305 2.325 2.345 2.340 2.263 2.271 2.323
2.248 2.246 2.310 2.255 2.268 2.213 2.384 2.273
2.103 2.117 2.095 2.182 2.123 2.138 2.121 2.193
2.138 2.110 2.142 2.302 2.129 2.124 2.137 2.361
1.443 1.472 1.443 1.483 1.450 1.485 1.452 1.489
1.478 1.475 1.470 1.420 1.466 1.470 1.450 1.416
1.295 1.300 1.300 1.269 1.298 1.300 1.305 1.262
the Pd−C1 distance is ranging from 2.263 to 2.374 Å, whereas the short Pd−C2 and Pd−C3 bonds are around 2.2 and 2.1 Å in length, respectively. The shorter bonds correspond to stronger interactions between the CC and Pd atoms. The short Pd−O bonds in the range of 2.110 to 2.142 Å indicate some strong interaction between the Pd and oxygen atoms, which leads to relatively distinct CO elongation. However, for η4-trans modes of both isomers, Pd−O distances above 2.3 Å are longer than that of other adsorption configurations, which indicate relatively weak Pd−O interactions. This finding explains the absence of distinct change on the CO length (around 1.260 Å) of η4-trans modes after adsorption. As a result of the interaction between Pd and 2-ethylhexenal, both CC and CO double bonds elongated to some extent, leading to substantial decrease of their bond strengths. Most of the CC bonds elongated above 1.450 Å and near 1.5 Å compared with the initial lengths of Z- and E-isomers at 1.356 and 1.353 Å. Similarly, the CO bond length has been elongated to around 1.3 Å with respect to the gas-phase reference of 1.2 Å (Figure 1), except in the η4-trans mode, which shows that Pd prefers the CC bond than the CO bond. Interestingly, as both double bonds are lengthened, C1−C2 bond lengths are simultaneously shortened to some extent (Table 1). The adsorption configurations of several E-isomers after optimization are shown in Figure 3. It can be seen that Pd−O distance in E-η4-trans (2.361 Å in Table 1), as shown in Figure 3d, is larger than those (around 2.1 Å in Table 1) in E-η3-cis (Figure 3a), E-η3-trans (Figure 3b), and E-η4-cis (Figure 3c).
and the oxygen atom also additionally interacts with the palladium atom. On η4-cis and η4-trans (Figure 2c and d separately), both double bonds were observed to simultaneously interact with Pd. The adsorption energy (Eads) is defined as the difference between the energy of the whole adsorption system (adsorbate + slab) and that of the bare slab and the isolated adsorbate as shown in eq 1, and thus, the most negative adsorption energy corresponds to the most stable adsorption configuration under this definition. The active energy barrier (Eact) is defined as eq 2. Eads = E(adsorbate/slab) − E(adsorbate) − E(slab)
(1)
Eact = E TS − Er
(2)
where ETS is the energy of the structure of transition state (TS), and Er is the energy of reactant.
3. RESULTS AND DISCUSSION 3.1. Adsorption of 2-Ethylhexenal on Pd(111). The adsorption of 2-ethylhexenal on Pd(111) was calculated; adsorption energy and structures are listed in Table 1. Adsorption energies of cis modes are observed to be lower than that of trans modes for both η3 and η4 configurations of both Z- and E-isomers, indicating that cis adsorption modes are relatively stable than trans. Moreover, η3-trans modes are slightly stable compared with η4-trans for both isomers based on the adsorption energy data in Table 1 (−64.3 vs −59.2 kcal/ mol and −71.7 vs −59.5 kcal/mol, respectively). Structurally, C
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Figure 3. Structures of 2-ethylhexenal adsorbed on Pd(111) after optimization ((a) E-η3-cis, (b) E-η3-trans, (c) E-η4-cis, and (d) E-η4-trans). Green and orange balls correspond to palladium atoms in the first layer and the lower three layers separately for easy recognition.
The η4-cis mode (Figure 3c) tends to transform into η3-cis (Figure 3a) after optimization. Furthermore, the distortion of 2-ethylhexenal molecule after adsorption onto Pd(111) surface was noted. The initially parallel adsorption of the propyl group attached to C3 has tilted away from the Pd surface. Meanwhile, the initially tilted adsorption mode of the ethyl group attached to C2 has become more tilted. These observations are consistent with previous reports on the adsorption of prenal (3-methyl-2-butenal), in which changes in intermolecular angles are attributed to the transformation of carbon orbital from planar sp2-like to a tetrahedral sp3-like hybridization.42 Only both double bonds as well as double bond atoms remain flatly adsorbed on the surface. 3.2. Overview of Hydrogenation Pathways of 2Ethylhexenal on Pd(111). As mentioned, the E-trans conformer is the most stable among various conformers in the gas phase. Nonetheless, cis modes are more energetically favored than trans modes according to Table 1. The reaction
barrier for the conversion of E-trans to E-cis with accompanying bond rotation is roughly 13.0 kcal/mol and for the conversion of E-trans to Z-trans above 66.9 kcal/mol because of bond cleavage in the gas phase. Further conversion from Z-trans to Zcis has an energy barrier of 9.5 kcal/mol. On Pd(111) surface, E-η3-trans mode is converted into cis mode by overcoming 19.0 kcal/mol active barrier. For E-η4-trans mode, the trans-cis is a two-step conversion: E-η4-trans mode converts itself into E-η3trans first, with the barrier of 2.0 kcal/mol, and then further forms the E-η3-cis mode with 19.0 kcal/mol. Whether in the gas or on the surface, conversion of E-trans into other configurations is difficult with high conversion energy barriers. Therefore, E-trans modes of η3 and η4 are selected as the initial configurations for the investigation of 2-ethylhexenal hydrogenation process. Figures 4 and 5 present the energy profiles of the hydrogenation of E-η3- and E-η4-trans-2-ethylhexenal to 2ethylhexanol using 2-ethylhexanal, respectively. The CC of 2D
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Figure 4. Energetic profiles on different pathways of E-η3-trans-2-ethylhexenal hydrogenation to produce 2-ethylhexanol via 2-ethylhexanal intermediate. For C3 → C2 → O → C1 pathway, the dissociated hydrogen atom bonds with C3 atom first, followed by C2, and then oxygen and C1.
Figure 5. Energetic profiles on different pathways of E-η4-trans-2-ethylhexenal hydrogenation to produce 2-ethylhexanol via 2-ethylhexanal intermediate.
ethylhexenal is first hydrogenated to form the saturated aldehyde, 2-ethylhexanal, followed by the hydrogenation of the CO group to generate the saturated alcohol, 2ethylhexanol. In the diagram, zero of energy corresponds to the energies of the 2-ethylhexenal (g) and 2H2 (g) reactants in the gas phase, along with the clean Pd surface. The whole reaction is a multistep process involving the coadsorption of 2ethylhexenal and molecular hydrogen, dissociation of hydrogen, and diffusion of atomic hydrogen toward the reactant species, in addition to the hydrogenation reactions. For simplicity, dissociated barriers of hydrogen are not explicitly shown in both figures. According to our calculations, the process of hydrogen dissociation to H atoms on Pd(111) needs less than 1 kcal/mol, which is lower than the 4.8 kcal/mol reported previously.43 Thus, the exothermic hydrogen dissociation and adsorption proceeded smoothly on Pd(111), which also reasonably agrees with experimental results.18,19 Moreover, the diffusion paths of the hydrogen atoms are also not explicitly demonstrated in both figures. As the reactant of subsequent hydrogenation steps, every state after hydrogen atom diffusion toward the target hydrogenation species is indicated by thick dashed lines. The 3-fold fcc or hcp hollow sites are found to be the most energetically preferred for the adsorption of dissociated hydrogen atoms with Pd−H distance from 1.79 to
1.82 Å, which is consistent with previous studies.44,45 Based on calculations, the diffusion from the hcp site to the adjacent fcc site and from fcc site to the neighboring hcp has energy barriers of 1.8 and 3.4 kcal/mol, respectively, that agrees well with the published data, in which the diffusion barriers of hcp to fcc and vice versa are calculated at 2.5 and 3.5 kcal/mol, respectively.46 Therefore, the hydrogen molecule can be assumed to have dissociated first and then adsorbed somewhere distant from the target molecule for hydrogenation; the dissociated H atom then approaches the molecule. As mentioned, hydrogen diffusion on Pd(111) between neighboring sites is associated with low energy barrier to facilitate the approach of the hydrogen atom toward the molecule through several diffusion steps; thus, the diffusion barriers of hydrogen atoms are not depicted in details. Only the overall reaction energy of hydrogen diffusion can be obtained from the data shown in Figures 4 and 5. Generally, this reaction energy is endothermic. The series of hydrogenation reactions of 2-ethylhexenal with E-η3-trans on Pd(111) will be discussed in details. From Figure 4, two hydrogen molecules are adsorbed dissociative on Pd(111) with coadsorption of 2-ethylhexenal first. The dissociative adsorption energy on Pd(111) of one hydrogen molecule is calculated as −28.75 kcal/mol, which is in agreement with the −20.8 kcal/mol value obtained from E
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The Journal of Physical Chemistry C experimental adsorption isotherms.47 Moreover, Dong et al.48 showed by DFT calculations using the Vienna ab initio simulation package that the adsorption energy of one H atom on the fcc 3-fold hollow position is −11.5 kcal/mol. From Figure 4, there are two possible routes for the CC hydrogenation of E-η3-trans-2-ethylhexenal to generate the intermediate 2-ethylhexanal depending on which carbon (C2 or C3) is hydrogenated first. The energy barriers for the formation of C3−H bond first followed by the C2−H bond are 18.4 and 5.3 kcal/mol, respectively (pathway C3 → C2). The energy barrier for the formation of C2−H bond first is 16.4 kcal/mol, which is slightly lower than 18.4 kcal/mol, whereas the subsequent formation of C3−H bond is 17.1 kcal/mol, which is significantly higher than 5.3 kcal/mol (pathway C2 → C3). It can be concluded that both C3 → C2 and C2 → C3 pathways are available for the production of the 2-ethylhexanal intermediate. Meanwhile, CO bond length of the 2ethylhexanal formed through C3 → C2 and C2 → C3 pathways is reduced to 1.231 and 1.298 Å, respectively, from the 1.300 Å bond length of E-η3-trans-2-ethylhexenal (Table 1). With regard to the hydrogenation of the intermediate, two routes are possible, depending on the bond (C1−H or O−H) that will be formed first. For the 2-ethylhexanal produced through the C3 → C2 pathway, the energy barriers for the formation of the O−H bond first followed by the C1−H bond to obtain 2-ethylhexanol are 11.8 and 16.1 kcal/mol separately, and the energy barriers for this opposite process that is the formation of C1−H bond first and then the further formation of O−H bond are 8.8 and 12.9 kcal/mol, respectively. Similarly, for the 2-ethylhexanal produced in the C2 → C3 pathway, the active barriers of the initial hydroxyl formation and then the C1−H bond are 13.7 and 16.4 kcal/mol separately, while that of this opposite process are 12.8 and 21.3 kcal/mol, respectively. These results indicate that either the initial hydroxyl formation or the initial hydrogenation of the carbonyl carbon atom to form the C1−H bond are favored to produce saturated alcohol. For the whole course to produce 2ethylhexanol, we note that formation of 2-ethylhexanal (the hydrogenation of CC) is slightly harder than the subsequent hydrogenation of CO, as shown in Figure 4. A possible explanation is in comparison with O and C1 atoms of CO, the steric hindrance that C2 and C3 atoms of CC experience from the molecule such as −CHO (aldehydegroup), −CH 2 CH 3 (ethyl group), and −CH 2 CH 2 CH 3 (propyl group), as shown in Figure 3, which makes it difficult for H to attack C2 and C3. Apparently, comparison of the energy barriers between C C hydrogenation and further CO hydrogenation to form saturated alcohol through different routes (Figure 4) shows that (1) whether C2 or C3 is first hydrogenated, the hydrogenation of CC to generate saturated aldehyde is favored through the C3 → C2 and C2 → C3 routes; (2) on the hydrogenation of CO of 2-ethylhexanal, either the initial hydrogenation of carbonyl carbon atom or oxygen atom is available; (3) based on E-η3-trans adsorption configuration, pathways C3 → C2 → O → C1, C3 → C2 → C1 → O, C2 → C3 → O → C1, and C2 → C3 → C1 → O are feasible for the formation of 2ethylhexanol. Meanwhile, structures of transition states through these four pathways for E-η3-trans are described in Figure 6a−d. In the hydrogenation of E-η4-trans-2-ethylhexenal, the calculated dissociative adsorption energy of one hydrogen molecule with simultaneous coadsorption of 2-ethylhexenal on Pd(111) is −25.4 kcal/mol, while it is −28.75 kcal/mol in the
Figure 6. Transition states on the hydrogenation 2-ethylhexenal over the Pd(111) surface: (a) E-η3-trans (C3 → C2 → O → C1), (b) E-η3trans (C3 → C2 → C1 → O), (c) E-η3-trans (C2 → C3 → O → C1), (d) E-η3-trans (C2 → C3 → C1 → O), (e) E-η4-trans(C3 → C2 → O → C1), and (f) E-η4-trans (C3 → C2 → C1 → O). The black dash lines connect the H atom on the Pd(111) surface with the carbon atom or the oxygen atom that would be hydrogenated.
hydrogenation of E-η3-trans. This difference can be accounted to the adsorption energy of hydrogen that is closely related to the adsorption sites on the Pd surface, as well as the distance between the dissociated hydrogen atom and the adsorbed 2ethylhexenal. Both fcc and hcp hollow sites are proven to be priority sites for hydrogen adsorption, although a 3.9 kcal/mol energy difference between these two sites still exist for the dissociative adsorption of one hydrogen molecule.45,46 Therefore, considering the factors mentioned above, some energy difference for the dissociative hydrogen adsorption on Pd(111) between E-η3-trans and E-η4-trans were allowed. Like the case of E-η3-trans mode, two routes are possible for the hydrogenation of E-η4-trans-2-ethylhexenal to form 2-ethylhexanal, the C3 → C2 and C2 → C3 pathways. Based in Figure 5, the energy barriers of each elementary step through the C3 → C2 route are 17.6 and 13.9 kcal/mol, respectively. The saturated aldehyde produced has CO bond length elongated to 1.310 Å from 1.262 Å of the adsorbed 2-ethylhexenal (Table 1). In the C2 → C3 pathway, C2 is hydrogenated first, with the active energy of 19.1 kcal/mol, followed by C3 by overcoming an energy barrier of 16.6 kcal/mol. In contrast with the favorable C2 → C3 and C3 → C2 routes for the formation of saturated aldehyde with E-η3-trans mode (Figure 4), production of 2ethylhexanal on E-η4-trans mode is also feasible with imposing a little difficulty, which could be attributed to the different F
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ethylhexenal and dissociation adsorption of hydrogen, as well as diffusion of H atoms on Pd(111) are, as mentioned, fast processes. Thus, the formation of 2-ethylhexanal intermediate and its subsequent hydrogenation to 2-ethylhexanol on the Pd surface determines the whole reaction process, which agrees well with the assumption above. Especially the steric hindrance f r o m d i ff e r en t g r o u p s ( −C H O , −C H 2 CH 3 , a n d −CH2CH2CH3) of 2-ethylhexenal molecule impedes the attacking of H adsorbed on Pd surface to C2 and C3 of C C to some extent, which increases the difficulty of C−H bond formation. In addition, considering the relatively small difference between the adsorption and desorption barriers, the desorption barrier was considered approximately equal to the absolute value of adsorption energy.49−51 The calculated desorption barrier of 2-ethylhexanal with 52.0 kcal/mol is higher than its hydrogenation barrier, as depicted in Figures 4 and 5, which demonstrates that the further hydrogenation of 2-ethylhexanal to saturated alcohol can proceed easily rather than escape into the gas phase. While the desorption barrier of the desired product 2-ethylhexanol was calculated as 55.4 kcal/mol. This high barrier quickly confines the desorption of 2-ethylhexanol from the Pd surface into the gas phase. Note, there is still a potential problem that 2-ethylhexenal, 2-ethylhexanal intermediate, and product 2-ethylhexanol would adsorb simultaneously and competitively on the Pd(111). Thus, the desorption and hydrogenation rates of the intermediate, the desorption rate of 2-ethylhexanol, as well as some operating conditions affecting both rates, should be considered significantly for the efficient production of 2-ethylhexanol.
adsorption modes of 2-ethylhexenal on Pd(111). The CC bond with E-η3-trans mode elongates more than with E-η4-trans (1.470 Å vs 1.416 Å in Table 1) after 2-ethylhexenal adsorption; thus, the CC bond of E-η3-trans is more activated and much easily hydrogenated. After the formation of 2-ethylhexanal through the C3 → C2 route, the active barrier of the hydroxyl formation first is 11.3 kcal/mol, which equals that of the initial C1−H bond formation. And then 2-ethylhexanol can be obtained by the further hydrogenation of carbonyl carbon atom with 7.2 kcal/ mol barrier, which is lower than 13.4 kcal/mol of O−H bond formation later in Figure 5. All these results indicate the hydrogenation of intermediate generated via C3 → C2 pathway to produce saturated alcohol is a smooth process, which resembles the case discussed in the reaction process on E-η3trans (Figure 4). However, it is difficult to get target 2ethylhexanol through the C2 → C3 route, considering that the aldehyde group of 2-ethylhexanal produced has become upright to and away from the Pd(111) surface, and hence, hydrogenation of the carbonyl group fails. Therefore, only C3 → C2 → O → C1 and C3 → C2 → C1 → O are easy reaction pathways among the different routes as described in Figure 5, and structures of transition states via both pathways for E-η4trans are depicted in Figure 6e,f. Comparing with the reaction routes of E-η3-trans and E-η4trans on Pd(111), hydrogenation of CC seems to be much slightly troublesome than hydrogenation of CO (Figures 4 and 5). As can be seen, the highest hydrogenation barrier of CC is higher than that of CO with less than 7 kcal/mol in the same reaction pathway except the C2 → C3 → C1 → O route in E-η3-trans mode, and some even less than 1 kcal/mol. Hence, the formation of 2-ethylhexanal is found to be relatively slow via several routes, and further hydrogenation to generate saturated alcohol is slightly rapid, although the former is not regarded as a rate-limiting step. Furthermore, the hydrogenation of 2-ethylhexanal on E-η4-trans was more feasible due to lower barriers in contrast with that of E-η3-trans. This is closely related to the various activation levels of CO with Eη3-trans and E-η4-trans: as earlier mentioned, CO bond lengths evolved into 1.231 and 1.298 Å after the formation of 2ethylhexanal through the C3 → C2 and C2 → C3 pathways separately for E-η3-trans, whereas CO bond lengths elongated to 1.310 Å through C3 → C2 for E-η4-trans, with the latter being more activated. This explains why CO hydrogenation of 2-ethylhexanal under E-η4-trans condition proceeds much smoothly. However, adsorption energies of both E-η3-trans and E-η4-trans modes are −71.7 and −59.5 kcal/mol (Table 1), and the E-η4-trans mode can convert itself into E-η3-trans with only 2.0 kcal/mol active barrier, and further trans−cis conversion is expensive on Pd(111), which indicates the 2-ethylhexenal structures will equilibrate to a predominant more stable E-η3-trans mode, and thus, several available reaction routes with E-η3-trans mode are primary to produce desired product in contrast with E-η4-trans mode. Experimentally, Niklasson19 found that Pd has a good selectivity for 2-ethylhexanal; he assumed that the surface reaction was the dominating rate-determining step. Our calculations show that, after 2-ethylhexenal adsorption on Pd(111), CC bond length elongated to 1.470 and 1.416 Å from 1.353 Å for E-η3-trans and E-η4-trans separately, and C O bond length to 1.300 and 1.262 Å separately from 1.225 Å, implying that Pd has preference to CC comparing to CO. Meanwhile, based on calculations above, the adsorption of 2-
4. CONCLUSIONS DFT calculations were employed to investigate the adsorption and hydrogenation of 2-ethylhexenal on Pd(111) surface to form the saturated alcohol 2-ethylhexanol through 2-ethylhexanal intermediate. All possible conformers with E- and Zisomers of 2-ethylhexenal in the gas phase were considered in the study of the adsorption behaviors of 2-ethylhexenal. The cis modes were found to be consistently stable compared with the trans in terms of energy. Simultaneously, ethyl and propyl groups attached to C2 and C3 were released from the Pd surface after optimization. Carbon and oxygen atoms of both CC and CO interact with Pd atoms to varying degrees, which further results in the elongation of both double bonds. During the hydrogenation process of 2-ethylhexenal, the formation of intermediate was found to be feasible through both C3 → C2 and C2 → C3 routes for E-η3-trans and E-η4trans; when it comes to generating target product 2ethylhexanol, the C2 → C3 route cannot proceed successfully for the E-η4-trans adsorption mode. Comparing with the C3 → C2 → O → C1 and C3 → C2 → C1 → O routes on E-η4-trans, C3 → C2 → O → C1, C3 → C2 → C1 → O, C2 → C3 → O → C1, and C2 → C3 → C1 → O routes on the E-η3-trans mode are primary, and the formation of 2-ethylhexanal may be slightly slower than the production of 2-ethylhexanol. Hydrogenation reactions of 2-ethylhexenal on Pd surface might determine the whole process starting from adsorption of reactants. Factors affecting reaction rate, such as the partial pressure of hydrogen and temperature, should be considered to obtain the target saturated alcohol. In addition, selection of active metals possessing good preference to hydrogenation of CC or CO should be further explored. Selection and use of several metals to improve the efficiency of 2-ethylhexenal G
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hydrogenation reactions to produce 2-ethylhexanol is also anticipated.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: 86-10-89739078. Fax: 8610-69724721. Author Contributions †
These authors contributed equally (J.J. and L.Z.).
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the supports by the National Natural Science Foundation of China (Grant Nos. 21176253, 21036008, 21236009, and 21476260).
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