Theoretical Study of the Catalytic Performance of Activated Layered

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Theoretical Study of the Catalytic Performance of Activated Layered Double Hydroxides in the Cyanoethylation of Alcohols Cristina Cuautli, Jaime S. Valente, José Carlos Conesa, Maria Verónica Ganduglia-Pirovano, and Joel Ireta J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10935 • Publication Date (Web): 13 Mar 2019 Downloaded from http://pubs.acs.org on March 17, 2019

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The Journal of Physical Chemistry

Theoretical Study of the Catalytic Performance of Activated Layered Double Hydroxides in the Cyanoethylation of Alcohols Cristina Cuautli,a,1 Jaime S. Valenteb, J. C. Conesac, M. Verónica Ganduglia-Pirovanoc, Joel Ireta*a.

aDepartamento

de Química, División de Ciencias Básicas e Ingeniería, Universidad

Autónoma Metropolitana-Iztapalapa. A. P.55-534. Ciudad de México. 09340. México.

bInstituto

1

Mexicano del Petróleo, Eje Central 152, Ciudad de México. 07730. México.

Present address: Centro de Investigaciones Químicas, Universidad Autónoma del

Estado de Morelos, Av. Universidad 1001, Cuernavaca. 62209, Morelos, México

ACS Paragon Plus Environment

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

cInstituto

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de Catálisis y Petroleoquímica. C/Marie Curie 2, Cantoblanco, 28049 Madrid.

Spain.

*[email protected],

Tel. +52 55 5804 6413

ABSTRACT

The layered double hydroxides (LDHs) containing Mg2+, Al3+ and OH (MgAl-OH), commonly known as meixnerite, meixnerite-like or activated LDH, act as catalyst in the cyanoethylation of alcohols, for which the catalytic activity depends on the LDH composition, particularly on the ratio R between the amount of Mg2+ and Al3+. It is known that MgAl-OH with R = 3 presents the largest catalytic activity when the reactants are methanol and acrylonitrile. To determine the molecular basis of such behavior, the adsorption of OH, CH3O, and H2O on the (001) surfaces of MgAl-OH with compositions


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R = 2, 3 and 3.5 is investigated using density functional theory. It is found that the peculiar catalytic activity of MgAl-OH in the methanol cyanoethylation reaction correlates with the capability of the MgAl-OH surface to stabilize CH3O —process in which hydrogen bonding plays a crucial role— and the reactivity of the adsorbed CH3O lone pairs.


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I. INTRODUCTION Layered double hydroxides (LDHs) are a group of natural and synthetic materials with versatile properties that make them suitable for a great diversity of technological applications.1,2 The general formula of LDHs is [M2+(1-x)M3+x(OH)2]An x/n·mH2O where M2+ and M3+ are metal cations, mainly from the third and fourth periods, An are anions from almost any organic or inorganic compound,3, 4 and x is the trivalent cation fraction, i.e. x = T/(D+T) where D and T are the amount of M2+ and M3+ respectively. Metal cations are located in the octahedral interstices of the brucite-like layers that form the LDHs. These layers bear positive charges compensated by An anions hosted along the interlayer space (Figure 1, left). LDHs of different compositions have been synthetized combining M2+ and M3+ cations of similar size. It has been found that the lower and upper limits for the proportion of M2+ cations with respect to the M3+ ones, R = D/T, are around 2 and 4, respectively. In synthesized materials with R values smaller than 2 or larger than 4, LDH and metal-oxide phases coexist.5 One of the applications of LDHs containing Mg2+ and Al3+ as cations is as catalysts, usually after a calcination process in which LDHs progressively suffer the loss of


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physisorbed water, interlayer water, interlayer anions, and layer hydroxyl groups. Above ~300 °C, the layer structure collapses, and at approximately 400 °C, the solid crystallizes as Mg(Al)O in a MgO rock-salt like structure, with the Al3+ cations evenly distributed. Mg(Al)O presents a relatively large specific area (> 200 m2g-1) and strong Lewis basic sites, namely, the O2 species.6, 7 The systems resulting from the calcination have the ability to recover their original layer structure upon either hydration or interaction with anions, property known as memory effect.3 When the LDH structure is recovered by hydration, OH anions are incorporated as the charge-balancing species8,9 and the Al3+ cations remain evenly distributed, as it has been shown employing nuclear magnetic resonance spectroscopy.10 The resulting LDH, [Mg(1-x)Alx(OH)2]OHx/nmH2O (thereafter MgAl-OH), is commonly known as meixnerite, meixnerite-like, or activated LDH. On the (001) MgAl-OH surface (thereafter MgAl-OHsurf, Figure 1, right), charge compensating anions may also be adsorbed. When surface anions are OH (thereafter MgAlOHsurf/OH), like in activated LDHs, such anions act as a Brönsted basic sites in catalytic reactions.8


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Figure 1. Left: bulk structure of MgAl-OH. Right: the (001)-oriented MgAl-OH surface with no anions adsorbed on it. Color code: Oxygen atoms in red, Hydrogen atoms in gray, Magnesium atoms in green, Aluminium atoms in blue. The O label indicates Oxygen atoms of subsurface hydroxyl groups. The OH label indicates surface hydroxyl groups.

Since few solids with Brönsted basic sites are found in nature, activated LDHs have been extensively studied. Particularly as a catalyst in several reactions of industrial interest, such as the aldol condensation,8,

11

the Claisen-Schmidt condensation,12 the

Knoevenagel reaction,13 and the cyanoethylation of alcohols.14,

15,

16,

17

In the

cyanoethylation of alcohols with acrylonitrile, compounds containing active hydrogen atoms are added to the double bond of acrylonitrile, most likely through the following mechanism: first the alcohol is deprotonated by a Brönsted basic site on the surface, next the resulting alkolxide anion is stabilized on a surface Lewis acid site, and finally the lone


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pairs of the alkoxide oxygen attack the acrylonitrile double bond.14 In industry this reaction is used for obtaining intermediate compounds involved in the production of organic substances and drugs. In this work, we aim for the atomistic understanding of the high catalytic activity of activated LDHs, especially in reactions like the cyanoethylation of alcohols. The LDH catalytic activity can be modulated by varying the R ratio. For example, the MgAl-OHsurf/OH catalytic activity in the 2-propanol cyanoethylation reaction decreases monotonously upon increasing R in the interval from 2 to 4. If methanol is used instead of 2-propanol, however, the activity initially increases with increasing R, reaches a maximum for R = 3, and decreases for larger R values.14 The catalytic activity trend in the 2-propanol cyanoethylation reaction is explained in terms of the strength of Brönsted basic sites on the MgAl-OHsurf/OH surface. Since the acidity of the alcoholic proton in 2propanol is low, the reaction limiting-step is expected to be the alcohol deprotonation by a surface Brönsted basic site.14 Thus, the stronger the Brönsted basic sites, the greater the catalytic activity. Indeed, it is known that Brönsted basic sites on LDH surfaces strengthen as R decreases. For the methanol cyanoethylation reaction, the catalytic


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activity trend is explained in terms of ability of the MgAl-OHsurf to stabilize the methoxide anion (CH3O): as the acidity of the alcoholic proton in methanol is significantly greater than in 2-propanol, the limiting step in the methanol cyanoethylation reaction should not be the alcohol deprotonation but the CH3O stabilization on a surface Lewis acid site.14 It has thus been hypothesized that the CH3O stabilization on MgAl-OHsurf with R = 3 should be the largest. Whether the latter is true or not is scrutinized in this work. To assess the CH3O stability on LDH surfaces, density functional theory (DFT) calculations are performed to investigate the energetics of the formation of surface CH3O species. It is found that this process is indeed energetically more favorable on the MgAl-OHsurf/OH surface with R = 3 than on MgAl-OHsurf/OH with R = 2 and 3.5, but by only 1.4 and 3.5 kcal/mol, respectively. Unexpectedly, it is found that molecular CH3OH adsorption is slightly more favorable than the CH3O formation, although by only 0.3, 1.2, and 1.9 kcal/mol on MgAl-OHsurf/OH with R = 2, 3, 3.5, respectively. Moreover, the energetics associated to the removal of an electron from the lone pairs of the adsorbed CH3O species is investigated as a crude estimation of the ability of such species to attack the acrylonitrile double bond. It is found that the reactivity of the adsorbed CH3O species on


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the LDHs with R = 3 and 3.5 is quite similar, but larger with respect that on the LDH with

R = 2. Based on these results, it is argued that the peculiar catalytic activity of MgAlOHsurf/OH system in the methanol cyanoethylation reaction results from the interplay between the CH3O stabilization and its lone pairs reactivity.

II. MODELS AND COMPUTATIONAL METHODS

All electronic structure calculations are performed using DFT in the Kohn-Sham formulation with periodic boundary conditions. The Perdew-Burke-Enzerhof (PBE)18 approximation to the exchange correlation functional is used, as it has been shown by several groups to adequately describe the structure and dynamical properties of LDH materials.19-28 Moreover, PBE is used in this work to be able to compare with previous results obtained by some of us and reported elsewhere on the anion-layer interactions in bulk MgAl-OH.27, 28 The projector augmented wave (PAW)29, 30 method is used together with a plane-waves basis set as implemented in the VASP code version 5.4.1.31-34 Calculations are performed with a plane-wave cutoff energy of 650 eV as well as 444


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and 441 Monkhorst-Pack k-point meshes for sampling the bulk and surface Brillouin zones, respectively. Isolated water and methanol molecules are calculated with the methodology described above, but using an orthorhombic supercell with a large vacuum region in the three lattice directions to avoid interaction between periodic images. The point is used for sampling the Brillouin zone in such cases. The bulk of MgAl-OH with R = 2, 3 and 3.5 is modeled with a single layer in the unit cell of [Mg2/3Al1/3(OH)2](OH)1/3 composition for R = 2, [Mg3/4Al1/4(OH)2](OH)1/4 for R = 3 and [Mg7/9Al2/9(OH)2](OH)2/9 for R = 3.5. Unit cells with the  and  lattice parameter equal to 90º and containing a single layer of the material lead to a layer stacking known as 1H.35 Geometry optimizations that include full optimization of all internal coordinates and lattice parameters lead to unit cells with the  and/or  lattice parameters that slightly differ from 90º (Table 1). The distortion of the  and  lattice parameters provokes the sliding of consecutive layers containing the hydroxyl groups with respect to a vector normal to the (001) plane, thus resembling a 3R layer stacking,35 structure that is believed to be the predominant one in LDH experimental samples. Lattice parameters of the optimized bulk structures are given in Table 1; interlayer anions are located in the energetically more


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favorable interlayer positions that were found after an exhaustive sampling that was previously reported.27 The positions of the hydroxyl groups and the interlayer anions in the MgAl-OH layers are such that there is no inversion symmetry along the [001] direction (see Figure S1 of the supporting material). Therefore, the bulk cleavage along the {001} planes generates polar surfaces. Moreover, leaving all surface anions on the same side of the generated crystal slice (cf. Figure 2a), leads to a spuriously large surface dipole in the direction of the vacuum which may provoke convergence problems in the DFT calculations. To avoid that, surface anions are initially located as depicted in Figure 2b. The latter implies building slab models with an even number of anions where half of the exposed (to the vacuum) anions are left at the top side (up-surface) of the slab, and the other half are located at the bottom side (down-surface). The resulting slab models do not have identical terminations and there is no inversion symmetry thus dipole corrections36 are applied to eliminate the still existing net dipole in the direction of the vacuum.37


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Table 1. Lattice parameters (lengths in Å and angles in degrees) for the optimized bulk structures used for building the surface models.

Parameter

2MgAl-OH

3MgAl-OH

3.5MgAl-OH

a

5.31

6.16

9.28

b

10.56

6.19

9.30

c

7.50

7.21

6.88



117.0

80.8

96.4



77.0

90.0

74.0



119.8

119.8

120.1


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Figure 2. Schematic representation of the periodic models. a) Bulk model. b) Supercell of the non-relaxed slab model. c) Supercell of the relaxed slab model. c and a unit cell vectors stand for the [001] and [100] directions, respectively.

That net dipole is, however, small; e. g. after the geometry optimization performed without the dipole correction, as explained below, the remaining net dipole is small (< 0.3 Debye). As the direction in which the potential is corrected must be orthogonal to the other two directions, the bulk is cleaved in such a way that the  and  lattice parameters of the unit cells used for modeling surfaces take the value of 90º. The MgAl-OHsurf (001) surface models with R = 2 and R = 3 are considered with 12 and 21 periodicity, respectively, and that with R = 3.5 with 11 periodicity (cf. Table 1 and Figure 3). A vacuum layer of ~10 Å along the c lattice direction of the supercells is found to be large enough for avoiding interactions between periodic images. To determine the necessary slab thickness for modeling the surfaces, the convergence of the anion-layer interaction with respect to the number of layers used in the surface model is investigated. The averaged anion-layer


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interaction is approximated as Eanion-layer = −Ecleav + Erlax, where Ecleav = ½ (Enrlax-slab –

N Ebulk) and Erlax = ½ (Erlax-slab – Enrlax-slab). Ebulk is the bulk energy corresponding to the bulk unit cell used for creating the surface slabs, Enrlax-slab is the energy of the non-relaxed slab supercell in which all atoms preserve their bulk positions (Figure 2b), Erlax-slab is the energy of the geometry-optimized slab supercell, which is calculated by allowing all atoms in the supercell to relax while keeping the supercell lattice parameters fixed, and N is the number of layers with bulk composition forming the slab. The factor 1/2 in the Ecleav and Erlax calculations accounts for the fact that two anion-layer contacts are cleaved for creating the slab. However, as already mentioned, the surface slabs are not symmetric, and thus, one should not expect the up-surface anion-layer interaction to be the same as the down-surface one and therefore Eanion-layer accounts for the average value of such interactions. Moreover, Eanion-layer also accounts for surface relaxation effects not necessarily connected to the relaxation of the surface hydroxyl groups. In any case, Eanion-

layer

should primarily account for the anion-layer interactions since the main geometry

change after relaxation is connected to the surface hydroxyl groups, as it is schematically presented in Figure 2c; the up-surface and down-surface OH orient perpendicularly to


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the surface. As it is shown below (Section III), two layers per surface unit cell slab are sufficient for obtaining anion-layer interactions converged. The resulting composition of the surface slabs/supercells are [Mg8/12Al4/12(OH)2](OH)4/12 for R = 2, [Mg12/16Al4/16(OH)2](OH)4/16 for R = 3 and [Mg14/18Al4/18(OH)2](OH)4/18 for R = 3.5. These compositions preserve the bulk stoichiometry. In the construction of the surface models for investigating the processes relevant to the formation of CH3O, two LDH layers, namely, a surface and a bulk-like layer are used (cf. Figure 2). The geometry optimization of the surface slabs is performed following a two-step procedure. First, geometry optimizations of the non-relaxed bulk-truncated slabs are carried out allowing for relaxations of all atoms in the surface layer, including the upsurface OH which are initially parallel to the slab (as depicted in Figure 2b), whereas all atoms in the bulk-like layer as well as the interlayer anions are kept fixed at their bulk positions; (hereinafter, this layer shall be simply referred as bulk layer). Also, the downsurface OH is allowed to relax, as it is found that in this manner the net dipole in the slab is further reduced. As a result of such relaxations, and in line with the relaxations observed when all atoms in the slabs were allowed to relax, the up-surface and down-surface OH


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orient perpendicularly to the surface (as is depicted in Figure 2c). In the second-step, the up-surface OH is placed at various sites such as on-top of each surface Al and Mg atoms as well as on O atoms belonging to subsurface hydroxyl groups. All tested positions are labelled in Figure 3. On each of these surface sites, OH is initially located perpendicular to the surface. In this step, surface geometry optimizations are carried out allowing for relaxations of all atoms in the surface layer, whereas the down-surface OH is kept fixed at the position and orientation obtained from the first relaxation step, and all atoms in the bulk layer as well as the interlayer anions are kept fixed at their bulk positions. The surfaces slabs for which the up-surface OH species are at their energetically most favorable positions are denoted by MgAl-OHsurf/OH. For investigating the CH3O formation on the MgAl-OHsurf/OH surfaces, the reaction energy for the MgAl-OHsurf/OH + CH3OH(g)  MgAl-OHsurf/CH3O·H2O process is investigated, where MgAl-OHsurf/CH3O·H2O indicates the coadsorption of CH3O species and water. Thus, it is considered that a surface OH anion transforms into water upon subtracting the alcoholic proton of the methanol and thus the resulting CH3O anion and water adsorb nearby, as it was proposed in Ref. 14. Therefore, in order to create the


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MgAl-OHsurf/CH3O·H2O systems with R = 2, 3, 3.5, first the up-surface OH species of the corresponding MgAl-OHsurf/OH systems is replaced by a water molecule, and then the CH3O species is placed on each of the unoccupied nearby adsorption sites, oriented in such way that its O atom forms hydrogen bonds with the surface hydroxyl groups. The water molecule is also oriented in such way that its O atom forms hydrogen bonds with surface hydroxyl groups and one of its OH bonds points towards the O atom of CH3O, so as to favor the formation of a hydrogen bond between the two coadsorbed species. Such water-methoxide anion configuration is found to be more stable than any other one in which hydrogen bonding between H2O and CH3O is not favored (Figure 4). It is considered that a hydrogen bond is formed if the distance between the proton and the acceptor atom is smaller than 2.5 Å and the acceptor-H-donor angle is larger than 135º. Then geometry optimization is carried allowing for relaxation of all atoms in the surface layer as well as the coadsorbed species. Adsorption configurations in which the O atoms of the adsorbates are closer than 2 Å are ruled out.


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Figure 3. Probed adsorption sites on the (001) surface of the MgAl-OH with a) R = 2, b)

R = 3 and c) R = 3.5, and (11), (21) and (11) periodicity, respectively. Hydrogen atoms are not shown for clarity. For color code see Figure 1.


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Figure 4. Schematic water-methoxide anion configurations on the (001) oriented MgAlOH surface. The upper-panel configuration is found to be more stable that the lowerpanel one. For color code see Figure 1. Hydrogen bonds are denoted by the dotted lines.

As a crude estimation of the methoxide lone pairs reactivity on the MgAlOHsurf/CH3O·H2O surfaces with R = 2, 3, 3.5 to attack the acrylonitrile double bond, the position with respect to the vacuum level of the p-orbitals connected to the lone pairs of the adsorbed CH3O is calculated for each system. The latter is achieved by calculating the atom- and orbital-projected local density of states (LDOS) on the O atom of the methoxide anion. To be able to compare the calculated LDOS of the different systems, the energy levels were referenced with respect to the corresponding vacuum level of each supercell in such a way that the calculated vacuum level is set to the zero of the energy scale. As it is shown in Figure S2 of the supporting material, the inclusion of dipole corrections allow us to estimate the vacuum level despite the (initial) polar nature of the slabs, and it also prevents spurious interactions between slabs through the vacuum.


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III. RESULTS To determine the adequate number of layers for building the MgAl-OHsurf (001) surface models (R = 2, 3, and 3.5), the convergence of the layer-anion interaction, Eanion-layer (cf. Sect. II), with respect to the number of layers in the slabs is investigated. The calculated values using one-, two- and three-layer slab models are listed in Table 2. According to these results, two-layer slabs are thick enough for obtaining converged layer-anion interactions within 0.2 kcal/mol with respect to the results obtained using three-layer slabs. Therefore, two-layer slabs are used in subsequent calculations. Considering that three hydrogen bonds are broken per surface OH in the three cases upon cleaving, and that in the three cases the surface OH still forms three hydrogen bonds upon relaxation, it is estimated that the anion-layer interaction strength per hydrogen bond varies between 18 kcal/mol (in MgAl-OHsurf with R = 3.5) and 21 kcal/mol (in MgAl-OHsurf with R = 3.5). These hydrogen bond strengths are comparable to the strength of the hydrogen bonds formed by OH with hydration water molecules, which amounts to 23.9 kcal/mol (at the


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coupled cluster level of theory with singles and doubles and perturbative triples excitations and extrapolation to the complete basis set limit).38 The hydrogen bonds formed by OH with surface hydroxyl groups should be of the same kind than those formed by this anion with water molecules, therefore the corresponding strengths should be comparable, as it is found here. This result gives us confidence in the methodology and surface models here employed for describing the interactions between MgAl-OHsurf and the adsorbed anions.

Table 2. Layer-anion interactions (in kcal/mol) estimated using MgAl-OHsurf surface models (R = 2, 3, and 3.5) built with N = 1, 2 and 3 layers with bulk composition.

R

N 1

2

3

2

61.4

60.5

60.3

3

60.2

62.7

62.5


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3.5

57.4

54.5

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54.6

Next, OH species are placed on all the surface sites indicated in Figure 3 to determine the energetically most favorable ones for each system. It is found that on MgAl-OHsurf/OH with R = 2 and 3, the most stable adsorption sites are on top of Mg atoms (M1 and M3 respectively), whereas on MgAl-OHsurf/OH with R = 3.5, it is over a subsurface hydroxyl group (O2). It is noticeable that surface Al atoms are not energetically preferred, indicating that interactions different than simple charge-charge electrostatic attractions crucially contribute to the anion-layer binding. Likely, these attractive interactions correspond to the hydrogen bonds formed by the anions with the layer hydroxyl groups.27 The difference in the OH adsorption energy with respect to the most stable adsorption site tends to increase upon changing the MgAl-OHsurf composition from R = 2 to R = 3 and R = 3.5 (Figure 5), with largest differences of about 10 kcal/mol. Several adsorption sites are energetically disfavored with respect to the most stable ones solely by few kcal/mol, about 2 kcal/mol or less, particularly for the MgAl-OHsurf with R = 3 system, for which, e.g., the O4 and O7 sites are by 0.5 kcal/mol less stable than the M3 site. This result suggests


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that the potential energy surface for OH adsorption on the MgAl-OHsurf with R = 3 presents flat regions like the one associated to the interlaminar anion in the MgAl-OH bulk with R = 3.27, 28

Figure 5. Differences in the up-surface OH adsorption energy with respect to the most stable adsorption site. For MgAl-OHsurf/OH with R = 2 (red bars), the most stable position is M1. For MgAl-OHsurf/OH with R = 3 (green bars) the most stable position is M3. For MgAl-OHsurf/OH with R = 3.5 (blue bars) the most stable position is O2. Notice that for some adsorption sites, only one or two bars appear owing to the absence of such adsorption sites on the corresponding system (cf. Figure 3).


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To investigate the energetics of the CH3O formation, CH3OH is added to the MgAlOHsurf/OH systems of lowest energy considering that the deprotonation of the alcohol has already happened and water has formed, and thus in the initial structures the adsorbed surface species are H2O and CH3O (Figure 4), which are nearby located as explained above (Sec. II). For the case of MgAl-OHsurf/OH with R = 3, CH3OH is also added to the system in which OH is adsorbed on top of O7 because it is solely 0.2 kcal/mol higher in energy than the most favorable structure with OH on M3. After relaxing the geometries for all three surfaces (R=2, 3, and 3.5), keeping the bulk-like layer atoms and the down-surface anion fixed (the latter at the positions obtained from the first-step in the geometry optimization of the MgAl-OHsurf slabs, see Section II), it is found that CH3OH and OH are the adsorbed species on the energetically most favorable relaxed structures (Figure 6, left); i.e., as result of the geometry optimization, the H2O proton involved in the hydrogen bond formation with CH3O moves back regenerating CH3OH. The overall process can be described by the reaction MgAl-OHsurf/OH + CH3OH(g)  MgAl-OHsurf/OH·CH3OH.


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The associated reaction energies (Ea) along with the most favorable adsorption sites for CH3OH and OH are listed in Table 3. Ea can also be considered as the methanol adsorption energy. It is found that CH3OH adsorption on MgAl-OHsurf/OH with R = 3 is slightly more favorable (by ~ 2 to 3 kcal/mol) than on LDHs with R = 2 and 3.5. Also noticeable is the OH adsorption site at final state; OH recovers the position it has at the MgAl-OHsurf/OH surfaces, namely, M1, O7 (comparable to M3) and O2 for R = 2, 3, and 3.5, respectively. Moreover, the reaction energy associated to the CH3O formation (Ef) with coadsorbed water (Figure 6 right), i.e., MgAl-OHsurf/OH + CH3OH(g)  MgAl-OHsurf/CH3O·H2O, is comparable to that of the CH3OH adsorption (Ea) (see Table 3). The difference between Ea and Ef, i.e. Ept = Ef  Ea, accounts for the energy required for the transfer a proton from CH3OH to OH on the surface. Ept increases with increasing R, being ~ 0 for R = 2, ~ 1 kcal/mol for R = 3, and ~ 2 kcal/mol for R = 3.5. It seems that the MgAl-OHsurf/OH·CH3OH

→

MgAl-OHsurf/CH3O·H2O (Figure 6) equilibrium exists, which

shifts to the left as R increases. For MgAl-OH with R = 2, this equilibrium implies solely a


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proton transfer, however for MgAl-OH with R = 3 and 3.5, adsorbed species must also diffuse towards their most favorable positions, as these are different in the MgAlOHsurf/OH·CH3OH structures as compared to those in the MgAl-OHsurf/CH3O·H2O ones (see Table 3). Nonetheless, it is noticeable that the CH3O formation is energetically more favorable on MgAl-OH with R = 3, as it was hypothesized, although solely by ~1.5 kcal/mol with respect to that on MgAl-OH with R = 2, but by ~ 3.5 kcal/mol as compared to MgAlOH with R = 3.5.

Table 3. Adsorption sites and reaction energies of CH3OH (Ea) and CH3O formation (Ef) in kcal/mol.

R

CH3OH adsorption

CH3O formation

OH a

CH3OHa

Ea

H2Oa

CH3O a

Ef

2

M1

A2

16.7

M1

A2

16.4

3

O7

O4

19.0

O6

O5

17.8


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3.5 aAdsorption

O2

O6

16.2

O6

A2

14.3

sites at the energetically most favorable structures

Figure 6. Adsorbates on the (001)-oriented MgAl-OH surface with R = 3. Left: CH3OH and OH. Right: H2O and CH3O. The depicted conformations of the adsorbates were obtained after the geometry relaxation. Carbon atom is depicted in pink, for the other atoms see the color code in Figure 1.

At this point, it makes sense to try to correlate the chemisorption properties of the LDH surfaces with the changes in the surface induced dipole upon adsorption. To this end, we have calculated the work function (using dipole corrections) for the examples of OH species and CH3O (without water to solely probe the methoxide reactivity) adsorbed on different sites of the MgAl-OH surfaces with R = 2 and 3, and, using the most stable site as reference, we have plotted the differences in the work function vs. the differences in the total energy (see Figure S3 of the


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supporting material) and found no correlation. The lack of correlation reflects that neither a charge transfer process nor a simple electrostatic interaction is driving the anion-surface interaction, but hydrogen bonding, as discussed above. As a crude estimation of the CH3O lone pairs reactivity in the MgAl-OHsurf/CH3O·H2O systems, the relative positions of the p-orbitals of the O atom in the CH3O species are compared (Figure 7). As the atom- and orbital-projected densities of states (LDOS) are referenced to the corresponding vacuum level in each system, the comparison presented in Figure 7 reflects the energy trend associated to the subtraction of one electron from the CH3O lone pairs, which may be correlated with their reactivity; i.e., considering absolute values, the lower the energy required for subtracting an electron, the larger the reactivity. According to these results, the lone pairs of MgAl-OHsurf/CH3O·H2O with R = 2 are the less reactive and those of MgAl-OHsurf/CH3O·H2O with R = 3, the most reactive (see inset in Figure 7).


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Figure 7. Projected density of states (LDOS) calculated for the p-orbitals of the oxygen atom in the CH3O species of the MgAl-OHsurf/CH3OH·H2O systems with R = 2 (green line), R = 3 (blue line), and R = 3.5 (orange line). The inset corresponds to a zoom of the higher energy region.

IV. DISCUSION According to the results presented above, the formation of CH3O species on MgAlOHsurf/OH (001) surfaces (R = 2, 3, and 3.5) is indeed energetically more favorable for the system with R = 3. Nonetheless, the existence of the MgAl-OHsurf/OH·CH3OH

→

MgAl-OHsurf/CH3O·H2O equilibrium that shifts toward the left as R increases, suggests that the catalytic activity of MgAl-OHsurf/CH3O·H2O with R = 2 should be the largest owing to a larger abundance of CH3O on that system as compared to the other two. However,


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the CH3O lone pairs reactivity trend indicates that in the MgAl-OHsurf/CH3O·H2O system with R = 2, lone pairs are the less reactive. Hence, one may expect that the consumption of CH3O by the reaction with the acrylonitrile will not greatly alter the MgAlOHsurf/OH·CH3OH

→

MgAl-OHsurf/CH3O·H2O surface equilibrium. Moreover, the

abondance of CH3O on MgAl-OHsurf/CH3O·H2O with R = 3 should be the second largest, but the CH3O lone pairs in that system are the most reactive. Therefore, the shift to the right of the surface equilibrium, owing to the CH3O consumption by the reaction with acrylonitrile, should be greater in MgAl-OH with R = 3 than with R = 2. Furthermore, the abundance of CH3O on MgAl-OHsurf/CH3O·H2O with R = 3.5 is expected to be the lowest, and the reactivity of its lone pairs is not so large as in MgAl-OH with R = 3. Hence, one should expect a lower catalytic activity of MgAl-OHsurf/CH3O·H2O with R = 3.5 than with R = 3. These comparisons are in line with the reported maximum in catalytic activity presented by MgAl-OH with R = 3 as compared to other smaller and larger R values.14 One may still ask why the formation of CH3O on MgAl-OHsurf/OH- with R = 3 is more favorable, and why its lone pairs are more reactive. Inspecting the optimized geometries


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of the MgAl-OHsurf/CH3O·H2O lowest energy structures, it is found that the O atom in CH3O of the MgAl-OHsurf/CH3O·H2O system with R = 3 gets slightly closer, by ~0.1 Å, to the surface as compared to the other two compositions. The latter result is obtained comparing the distances between the O atom in CH3O and the closest Al atom along the surface normal vector. Moreover, CH3O forms three hydrogen bonds with the surface hydroxyl groups on the MgAl-OHsurf/CH3O·H2O surface with R = 3, but only two on the other two surfaces. It seems that the hydroxyl groups arrangement on MgAl-OH with R = 3 is such that CH3O can be better accommodated on that surface to form more hydrogen bonds and to get closer to it. Certainly, many factors present in the real experiment have not been taken into account in our study, such as temperature, solvent, surface defects, polymorphism etc. Nevertheless, our results are valuable as a guide for the microscopic understanding of the reactivity of LDHs in the cyanoethylation of alcohols.

V. CONCLUSIONS


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In this work we have investigated the microscopic origin of the catalytic activity of MgAlOH LDHs in the cyanoethylation reaction of alcohols. By means of DFT calculations employing a periodic surface model in which half of the charged species on one side of the slab were moved to the other side, effectively reducing the net surface dipole, we have investigated the adsorption of OH, CH3O and H2O on the (001) surface of the MgAl-OH LDH with divalent to trivalent cation ratios of R = 2, 3 and 3.5. Based on the detailed analysis of the calculated geometries, electronic structures as well as adsorption and reaction energies, we conclude that the catalytic activity of MgAl-OH surfaces in the methanol cyanoethylation of acrylonitrile results from the subtle interplay between their ability to fix CH3O species and the capability of the CH3O lone pairs for attacking the acrylonitrile double bond. The latter may be the explanation for the larger catalytic activity of MgAl-OHsurf/CH3O·H2O system with R = 3 with respect to that of comparable systems with R = 2 and 3.5.

AKNOWLEDGEMENTS


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CC acknowledges CONACYT for the fellowship granted. Authors gratefully acknowledge the computing time granted by LANCAD and CONACYT in the supercomputer Yoltla at the LSVP at UAM-Iztapalapa.

SUPPORTING INFORMATION AVAILABLE.

Schemes of the bulk unit cell of MgAl LDHs depicting the common layer stacking presented in these materials. Averaged electrostatic potential along the [001] direction of the MgAl-OHsurf/OH systems with R = 2, 3 and 3.5. Changes in the systems’ total energy (E) with respect to changes in the work function (wf) upon changing the position of the adsorbed species.

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TOC


41


Left: bulk structure of MgAl-OH. Right: the (001)-oriented MgAl-OH surface with no anions adsorbed on it. Color code: Oxygen atoms in red, Hydrogen atoms in gray, Magnesium atoms in green, Aluminium atoms in blue. The O label indicates Oxygen atoms of subsurface hydroxyl groups. The OH label indicates surface hydroxyl groups. 85x39mm (300 x 300 DPI)


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Schematic representation of the periodic models. a) Bulk model. b) Supercell of the non-relaxed slab model. c) Supercell of the relaxed slab model. c and a unit cell vectors stand for the [001] and [100] directions, respectively. 85x40mm (300 x 300 DPI)



. Probed adsorption sites on the (001) surface of the MgAl-OH with a) R = 2, b) R = 3 and c) R = 3.5, and (1×1), (2×1) and (1×1) periodicity, respectively. Hydrogen atoms are not shown for clarity. For color code see Figure 1. 59x85mm (300 x 300 DPI)


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Schematic water-methoxide anion configurations on the (001) oriented MgAl-OH surface. The upper-panel configuration is found to be more stable that the lower-panel one. For color code see Figure 1. Hydrogen bonds are denoted by the dotted lines. 79x59mm (300 x 300 DPI)



Differences in the up-surface OH− adsorption energy with respect to the most stable adsorption site. For MgAl-OHsurf/OH− with R = 2 (red bars), the most stable position is M1. For MgAl-OHsurf/OH− with R = 3 (green bars) the most stable position is M3. For MgAl-OHsurf/OH− with R = 3.5 (blue bars) the most stable position is O2. Notice that for some adsorption sites, only one or two bars appear owing to the absence of such adsorption sites on the corresponding system (cf. Figure 3) 149x49mm (300 x 300 DPI)


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Adsorbates on the (001)-oriented MgAl-OH surface with R = 3. Left: CH3OH and OH−. Right: H2O and

CH3O−. The depicted conformations of the adsorbates were obtained after the geometry relaxation. Carbon atom is depicted in pink, for the other atoms see the color code in Figure 1. 85x39mm (300 x 300 DPI)



TOC 82x44mm (300 x 300 DPI)


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Theoretical Study of the Catalytic Performance of Activated Layered

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