Nickel Coordination to Lattice Oxygens in Basic LSX, X and Y Sodium

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Nickel Coordination to Lattice Oxygens in Basic LSX, X and Y Sodium Faujasites: A DFT Study H. Guesmi,*,† D. Costa,‡ D. Berthomieu,§ and P. Massiani† †

UPMC—Universite Pierre et Marie Curie, Laboratoire de Reactivite de Surface, UMR 7197 CNRS, 4 place Jussieu, Casier 178, 75252 Paris, France ‡ Chimie-ParisTech—ENSCP, Laboratoire de Physico-Chimie des Surfaces, UMR 7045 CNRS, 11 rue Pierre et Marie Curie, 75005 Paris, France § MACS—Institut Charles Gerhardt, UMR 5253 CNRS/ENSCM/UM2/UM1, 8 rue de l'Ecole Normale -34296 Montpellier cedex 5, France

bS Supporting Information ABSTRACT: The location and coordination of nickel cations inside the hexagonal prism of dehydrated sodium faujasites have been investigated as a function of the framework Al content and distribution by density functional theory (DFT) calculations. Three different Si/Al ratios have been considered in order to represent the Na-LSX (Si/Al = 1), Na-X (Si/Al = 1.4), and Na-Y (Si/Al = 2.0) faujasites that have the same framework topology but different basic properties. For each system, the most representative Al repartitions among the framework tetrahedral sites have been taken into account and their related preferred Ni2þ configurations have been established. Upon decrease of the Al content, the Ni2þ coordination changes from a perfect octahedral environment with Ni2þ located in the center of the hexagonal prisms (Ni/ Na-LSX) to five 4-fold and four 3-fold coordinations in Na-X and Na-Y faujasites, respectively. The preferred locations, inside the hexagonal prism, are those where Ni2þ can bind the highest number (highest coordination) of basic framework oxygen atoms even though the related Ni-O distances simultaneously increase. All configurations are associated to a significant distortion of the hexagonal prism as compared to the host Na-faujasite cluster which illustrates the zeolite flexibility. While 4-fold coordinated nickel weakly interacts with the framework Yfaujasite, it induces a stronger framework distortion. The monitoring effect of lattice oxygen atoms toward Ni-framework interaction and framework deformation as well as Ni2þ coordination and location inside zeolites are rationalized.

1. INTRODUCTION Zeolites are crystalline aluminosilicates well-known for their organized networks of micropores, their cationic exchange capacities and their acidic and basic active sites that make them particularly attractive for a large number of applications in adsorption, separation and heterogeneous catalysis. One of the key features of zeolites is their chemical composition that can be tuned as to answer the desired application. Generally speaking, the term zeolite refers to microporous crystalline aluminosilicates whose framework is built from the association of tetrahedral Si and Al atoms connected via oxygen atoms as to form bridging (O-)3T-O-T(-O)3 units constitutive of the lattice (T = Si or Al). The presence of trivalent (Al3þ) aluminum atoms in the lattice generates framework negative charges that are neutralized by charge compensating cations, most often alkali (adsorption, dehydration, exchange, basic catalysis) or protons (acidic catalysis). This narrow relationship between tetrahedral Al atoms and cationic sites renders the framework Al content determining toward both the cationic exchange capacities of zeolites and their acido-basic properties that depend on the number of r 2011 American Chemical Society

exchangeable protons (Brønsted acid sites) and on the framework negative charges with electron donor propensity (basic sites). Acidic zeolites have been extensively studied for long due to their major industrial impact in refining and petrochemistry. In contrast, the interest for basic zeolites is more recent but it has kept growing in the near past due to an increasing demand in basic materials able to answer needs in domains such as environmental-friendly fine chemistry,1-4 catalytic air treatment,5,6 or gas storage.7,8 In these basic solids, it is known that the higher the Al content and the counterion electronegativity, the stronger the strength of the basic sites (for detailed references on basic zeolites, see Barthomeuf’s review9). An expected consequence of zeolite basicity in heterogeneous catalysis is to change the catalytic route followed by reaction as compared to the same zeolite used in its acidic (protonic) form, thus potentially leading to distinct reaction products and to different activity and selectivity levels. Besides, lattice basicity can modify the Received: August 27, 2010 Revised: February 8, 2011 Published: March 09, 2011 5607

dx.doi.org/10.1021/jp108172t | J. Phys. Chem. C 2011, 115, 5607–5618

The Journal of Physical Chemistry C properties of active metal nanoparticles dispersed in the pores through electronic surface enrichment.10-13 It can similarly affect the active properties of extra-framework transition metal (TM) ions that participate themselves to charge compensation, being therefore directly influenced by the basic site features. Experimental evidence of the influence of framework basicity on exchanged divalent cations (Cu2þ, Zn2þ, Ni2þ, Co2þ, etc.)14-24 in zeolites have been widely reported. For instance, spectroscopic studies have shown that the distribution, coordination and activity of Cu2þ ions in low silica14-19 and high silica zeolite matrices20,21 vary with change of the structure type and Al content. Also, it has been reported that a decisive factor controlling the activity of cobalt ions in different zeolite structures is the negative charge of the zeolite framework.23 Nevertheless, the location of TM ions in zeolites and the characteristics of their interactions with the chemical neighboring often remain a matter of debate.15,24 This is especially the case for isolated divalent nickel cations exchanged in dehydrated faujasites (Ni2þ/FAU). Ni2þ-exchanged zeolites, especially Y-faujasites, are frequently employed not only for the reduction of NOx25-27 but also for the hydrogenation28 and oxidation of CO.29,30 Because knowing the properties of the Ni2þ sites is essential to understand the catalytic behavior of Ni2þ/zeolites, various studies have been conducted to identify the precise location as well as the intrinsic electronic properties of nickel in faujasite. Actually, the first attempts to locate Ni2þ in Ni/Na-FAU started 40 years ago31 and various types of experimental approaches have been applied since then. It is well established that Ni2þ in hydrated samples is located in supercages where it is hexacoordinated to water molecules32-35 through Ni-O bonds which length is around 2.06 Å.36-38 Upon dehydration, the Ni2þ ions migrate to more confined environments in order to optimize their coordination, as is classically observed for ions in zeolites.39 For low nickel content faujasites, the preferred Ni2þ sitting is then referred as the so-called SI sites, in the center of hexagonal prisms.32,33,38,40-42 However, important discrepancies on reported Ni-O distances and Ni2þcoordination numbers (vide infra) exist, depending on the study and on the used experimental technique.31-45 Since Ni2þ is a d8 ion of medium size exhibiting an intermediate hardness, it may easily accommodate different coordination numbers and symmetries. In addition, the lattice Al environment could play a key role on the nickel preferable location. Such factors and their unclear relationship make quite complex the understanding of the nickel behaviors and properties by experimental approaches. One attractive solution to correctly address the above problem is the density functional theory (DFT) approach that is indeed recognized as a very successful tool to study TM interactions in zeolites.14,15,46-51 Owing to constant technological improvements of calculators, the DFT method can nowadays be applied within a cluster approach for systems big enough to correctly represent TM ions and their local environnement.24 The aim of the present work is to rationalize, through a molecular description, the effect of the local framework basicity of zeolite faujasite toward the nickel location, coordination and interaction. Our interest is focused on the specific case of fully dehydrated faujasite having limited Ni contents, for which the preferred Ni2þ location—and even sole one for very low Ni contents (few wt%)—is inside the double 6-membered ring.38,42 Therefore, we propose a DFT investigation within a cluster approach of the influence of the framework Al content—and framework Si/Al distribution—on the configuration and sitting

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Figure 1. Schematic representation of the faujasite structure (adapted from ref 59) showing the widely referenced cationic positions (SI, SI0 , SII, and SIII sites). The enlarged part of the zeolite indicates the calculated cluster, which was cut off from the faujasite unit cell. O1, O2, O3, and O4 are the crystallographic lattice oxygen positions.

of Ni2þ within the hexagonal prism. A series of three basic Nafaujasites (FAU structure) with increasing Al content were considered, consisting of the well-known Y (Si/Al = 2.0), X (Si/Al = 1.4), and LSX (Si/Al = 1.0) zeolites. The LSX faujasite —often denoted “low silica X”—contains an equal number of tetrahedral Al and Si atoms within the framework, thus having the most regular Si-Al distribution in the faujasite series. In spite of this specificity, the bibliography for LSX is much poorer than that for other faujasites because its successful synthesis was only discovered recently.52-55 The paper is organized as follows: first, we present all calculated DFT configurations and the Ni2þ locations predicted in the three modeled faujasite types. Second, by considering the most probable—i.e., energetically stable—configurations, we analyze the nickel coordinations and dNi-O distances expected from such calculations and we discuss them in view of the experimental data available from literature. Then, the nickelframework interactions as well as the framework deformations are investigated. Finally, the relationship between the adopted nickel coordinations and the role played by framework basicity are highlighted.

2. COMPUTATIONAL DETAILS This section details the different host Na-FAU cluster models taken into consideration and the calculation methods used for optimization of the Ni2þ/Na-FAU systems. 2.1. Na-FAU Models. The crystal structure of sodium-LSX with Si/Al ratio of 1 was first considered since it is the most regular and isotropic system in the Al-containing faujasite series, characterized by a perfect succession of AlO4 and SiO4 tetrahedra, in accordance with the Lowenstein’s Al-O-Al avoidance rule.56 The chemical composition Si96Al96Na96O384 was modeled by considering the geometric parameters of the LSX unit cell found by Feuerstein et al.55 The placement of the extra-framework cations among the different crystallographic sites was modeled by selecting the distribution defined by Vitale et al.,57 5608

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The Journal of Physical Chemistry C

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Figure 2. DFT used framework clusters representing the hexagonal prism (balls and bond type) and a part of its environment (tube) for the (a) Na-LSX, (b) Na-X, and (c) Na-Y faujasites. The red, blue, and white balls are oxygen, silicon and aluminum atoms, respectively. The fixed hydrogen terminations are represented by the round ends. For clarity seek, only the extra-framework sodium cations located in SI0 site are presented by big yellow balls, other sodium cations are visible as small ones. Dashed circles represent anisotropic parts of the framework where Si atoms substitute Al ones (as indicated by arrows).

in which 32 Naþ ions are in site SI0 in the sodalite cages, facing the 6-membered rings (6-mr) of hexagonal prisms, 32 Naþ ions are in site SII, facing the 6-mr of sodalite cages toward the supercages, and 32 Naþ cations are in site SIII, in the plane of the 12-ring (12-mr) windows (Figure 1). Then, a cluster of 120 atoms, H24O60Na12Al12Si12 representing the hexagonal prism and a part of its environment (illustrated by the enlarged part of Figure 1) was cut out from the modeled Na-LSX zeolite lattice. All dangling bonds were saturated with H atoms (OH terminations set to 1.0 Å and oriented toward the direction of the next tetrahedral site). We denote 6-mr(xAl) with x = 1, 2, or 3 the sixrings forming the hexagonal prism and having xAl atoms. Considering the Na-LSX cluster (Figure 2a), it can be noted that the two 6-mr(3Al) are equivalent, being both formed by three Al and three Si as tetrahedral atoms connected by bridging oxygen atoms in O2 and O3 framework positions (Figure 1). Due to their similar chemical and structural environment, all O3 (respectively O2) oxygen atoms have similar basicity.58 The Na-X cluster (H24O60Na10Al10Si14) representing sodium faujasite with Si/Al ratio of 1.4 was obtained from the above reference Na-LSX model by substituting two Al by two Si. The first substitution was made in one of the two 6-mr of the hexagonal prism, thus giving a total of 5 Al and 7 Si atoms in the prism (i.e., the expected atomic Si/Al ratio of 1.4). The second substitution was made outside the prism and we chose to

position it close to the first substitution (upper part of the cluster in Figure 2b). This choice was made from preliminary calculations (not reported) that showed that two remote substitutions in NaX would result in a too high “dilution” of the substituted sites with very weak effect of the applied anisotropy on TM location. Therefore, it is the configuration representing a Si side enrichment (Figure 2b) that was chosen. Finally, two additional Naþ close to substituted Al atoms were removed for the sake of charge neutrality, and they were taken from SIII sites that are known as energetically less favorable than SI0 and SII sites, as is reviewed elsewhere.46 For the Na-Y faujasite system (H24O60Na8Al8Si16) with Si/ Al ratio of 2, the Na-LSX cluster was again taken as the initial reference model but three different configurations had to be distinguished in order to represent all possible Si-Al substitutions in the hexagonal prism. In all three systems, 2 Al atoms among the 6 possible ones constituting the prism were substituted by 2 Si atoms, thus keeping the Si/Al ratio of 2 in the prism (total of 8 Si and 4 Al), and two additional substitutions (overall Si/Al ratio of 2 for the all cluster) were made at the exterior part of the prism. In all cases, the number of Na ions was adjusted to ensure cluster charge neutrality. The three different Na-Y cluster types thus built are denoted A, B, and C (Figure 2c). In Na-Y cluster A, all substituted tetrahedral Al sites are positioned on the same side of the cluster. The 5609



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hexagonal prism is then vertically divided along the 3-fold axis into one “Al-poor” and one “Al-rich” regions (encircled part in Figure 2c-A). In cluster B, the four substituted Al sites are symmetrically selected with respect to the center of the hexagonal prism (2 opposite positions in both 6-mr(2Al) of the hexagonal prism and 2 sites in the respective external coordination neighboring, as is seen by the encircled upper and bottom parts of the cluster in Figure 2c-B). In the last configuration, i.e., cluster C, the two Al substitutions in the hexagonal prism are made in the same 6-mr, the other ring being untouched, thus leading to an hexagonal prism horizontally divided into one Alpoor and one Al-rich regions. In such configuration, the 6-mr(1Al) belongs to a low-Al content part of the cluster (encircled upper part in Figure 2c-C), whereas the other 6-mr(3Al) belongs to a LSX-like part of the cluster. 2.2. Calculation Methods. All density functional theory (DFT) calculations were performed using the Gaussian 03 package.60 Geometry optimizations were carried out using the Becke3-Lee-Yang-Parr (B3LYP)61,62 hybrid functional. Basis sets at the 6-31G* level were used for the Si, Al, O, Na, and H atoms. Ni was described using the extended 6-311þG* basis set. To justify our choice, the same level of theory was applied for the optimization of the [Ni(H2O)6]2þ molecule. The resulting bond length value of 2.06 Å was in fair agreement with the experimental dNi-O distances in Ni-OH2 bonds measured in hydrated faujasites (2.04-2.06 Å).36-38 All geometries were optimized with an energy convergence criterion of at least 10-7 Ha and a maximum norm of the Cartesian gradient of 10-4 Ha/Bohr. No corrections were made with respect to the basis set superposition error (BSSE).63 During optimization, all atoms were allowed to relax except for the H terminations that were kept frozen through all subsequent calculations. These fixed hydrogen atoms were positioned over the third coordination sphere of the hexagonal prism, far enough to consider the absence of effect of constraints that can be a source of artifacts in the cluster approach.64 The initial calculations were made on fully Naþ exchanged faujasites with the aim to optimize the parent host Na-FAU models and to consider the most realistic ones, based on experimental data. The geometric parameters (bond distances and bond angles) obtained for the considered Na-LSX, Na-X, and Na-Y clusters are provided in the Supporting Information, where they are discussed and compared to available experimental results.35,39,65-74 The good agreement between these calculated values and the bibliographic data shows the correctness of the

cluster models and supports their validity as host systems for Ni2þ. Therefore, an isolated Ni2þ cation was next introduced in each of the considered Na-FAU models by the substitution of two Naþ cations initially located inside the hexagonal prism. Energy optimizations of nickel structures at the singlet and at the triplet spin states show the stability of the high spin electronic state of nickel Ms = 3. Thus, only DFT results considering nickel in its triplet state are reported hereafter. The comparison of the structure energies for the various Ni locations was made by calculating, for each Ni/Na-FAU system, the relative energy ΔEconfig as the energy difference between the considered configuration and the most stable one for this system, taken as reference.

3. RESULTS AND DISCUSSION All Ni/Na-FAU configurations that led to a minimum on the Potential Energy Surface (PES) are described in section 3.1. For each configuration, the Ni-O and Ni-T distances are listed (Tables 1-3). Among the dNi-O distances, only those below 2.5 Å were considered to represent the first Ni2þ coordination sphere and to evaluate the related average dNi-O values. The front and upper views of all optimized systems, restricted for the sake of clarity to the hexagonal prism, are shown in Figures 3, 4

Figure 3. Minimum energy configurations of Ni sitting within the hexagonal prism of Ni/Na-LSX (6 Al and 6 Si at tetrahedral lattice positions): upper and side views of SI site (a) and SI0 6-mr(3Al) type site (b). The red, blue, white, and green balls are oxygen, silicon, aluminum, and nickel atoms, respectively.

Table 1. Ni2þCoordination and Distances (Ni-O and Ni-T) for the Preferable Nickel Locations in Ni/Na-LSX (Si/Al = 1)a bond distances (Å) site SI

SI0 6-mr(3Al)

coordination and (average dNi-O)

dNi-O3 (Å)

dNi-O2 (Å)

dNi-Al (Å)

dNi-Si (Å)

6 (2.29 Å)

2.23

3.60

3.42

3.41

2.24 2.24

3.63 3.67

3.44 3.44

3.42 3.43

2.24

3.68

3.45

3.43

2.37

3.70

3.51

3.47

2.44

3.72

3.55

3.61

2.01

2.12

2.91

2.82

2.02

3.41

3.32

3.35

2.02

3.50

3.42

3.38

4 (2.04 Å)

ΔEconfig. (kcal mol-1)

0.0

þ8.0

a The coordination and related average distances (in brackets) are established from the dNi-O distances below 2.5 Å (in italics). ΔEconfig. refers to the relative stability of configuration regarding the most stable one for Ni/Na-LSX.

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Figure 4. Minimum energy configurations of Ni sitting within the hexagonal prism of Ni/Na-X (5 Al and 7 Si at tetrahedral lattice positions): upper and side views of SINi/Na-X type site (a), SI0 6-mr(3Al) type site (b), and SI0 6-mr(2Al) type site (c). The red, blue, white, and green balls are oxygen, silicon, aluminum, and nickel atoms, respectively.

Table 2. Ni2þCoordination and Distances (Ni-O and Ni-T) for the Preferable Nickel Locations in Ni/Na-X (Si/Al = 1.2)a bond distances (Å) site


dNi-O3 (Å)

dNi-O2 (Å)

dNi-Al (Å)

dNi-Si (Å)

SINi/Na-X

5 (2.23 Å)

2.05

3.37

3.18

3.28

2.19

3.57

3.39

3.30

2.21 2.26

3.61 3.62

3.47 3.56

3.42 3.48

2.45

3.81

3.58

3.69

3.23

3.81


þ0.6

3.88 3.94

SI0 6-mr(3Al)

SI0 6-mr(2Al)

4 (2.05 Å)

4 (2.05 Å)

1.96

2.20

2.92

1.95

3.37

3.36

3.34

2.07

3.43

3.42

3.36

1.98 2.03

2.06 3.36

2.90 3.37

2.84 3.36

2.11

3.40

2.84 0.0

þ2.4

3.37 3.41

The coordination and related average distances (in brackets) are established from the dNi-O distances below 2.5 Å (in italics). ΔEconfig. refers to the relative stability of configuration regarding the most stable one for Ni/Na-X. a

and 5. Moreover, in the case of the SI0 type sites, denoted SI0 6-mr(xAl) (x = 1, 2, or 3), the upper views are limited to the 6-mr where nickel is localized. 3.1. Nickel Locations Inside the Hexagonal Prism. In the highly symmetrical LSX system, the geometry optimization predicts an energetically favorable Ni2þ location in site SI (Figure 3a). In this configuration, nickel is located in the center of the hexagonal prism and is coordinated to all six oxygen at O3 positions in the prism. Both 6-mr(3Al) of the prism are thus involved in the Ni2þ coordination sphere. A second possible configuration with PES minimum exists in LSX, but energetically much less favorable (Table 1), being the configuration where nickel is located in one of the two available SI0 6-mr(3Al) sites (Figure 3b). Compared to Ni/Na-LSX, Ni/Na-X presents some anisotropy since the hexagonal prism is constituted by one 6-mr(3Al) and one 6-mr(2Al). The DFT energy optimization predicts a high stability for both 5-fold and 4-fold Ni2þ coordinations (Table 2).

The 5-fold nickel coordination is related to a site hereafter denoted SINi/Na-X, positioned inside the hexagonal prism but shifted by 0.23 Å from the conventional centered SI site toward the high Al content region (Figure 4a). Although it permits Ni2þ coordination to a higher number of lattice oxygen atoms (2 and 3 oxygen atoms at O3 positions in the upper and bottom 6-mr, respectively), this configuration is found to be energetically competitive (ΔEconfig = þ0.6 kcal.mol-1, Table 2) with respect to the configuration in which Ni2þ is 4-fold coordinated in site SI0 6-mr(3Al). In such Ni2þ location, the divalent cation is stabilized in the plane of the six-membered ring where it interacts through Ni-O bonds significantly shorter than above (Table 2), with three and one oxygen atoms in O3 and O2 positions, respectively (Figure 4b). Short bond lengths are also recorded for the third possible Ni2þ location at site SI0 6-mr(2Al), i.e. in the plane of the 6-mr containing two Al tetrahedral; this last configuration is energetically less favorable (Table 2). 5611



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Figure 5. Minimum energy configurations of Ni sitting within the three different possible clusters representing the hexagonal prism in Ni/Na-Y (4 Al and 8 Si at tetrahedral lattice positions): upper and side views of SINi/Na-Y(A) (a) and SI0 6-mr(2Al) type sites (b) in cluster A; SINi/Na-Y(B) type site (c) in cluster B; SI0 6-mr(3Al) type sites (d,e) in cluster C. The red, blue, white and green balls are oxygen, silicon, aluminum and nickel atoms, respectively.

For Ni/Na-Y, several possible Ni2þ sittings and coordinations are found, depending on the cluster type (Table 3). The general behavior, as expected from basicity trends, is that nickel in all models tends to stabilize in the Al-richest region. In the anisotropic model A that involves one high Al content side (left side of the hexagonal prism in Figure 5a), the nickel cation is preferentially located inside the hexagonal prism in a site denoted SINi/Na-Y(A), which is shifted from the center of the hexagonal prism by 0.9 Å toward the higher Al-loaded side of the prism. At this position, Ni2þ makes 3 bonds with two and one oxygen atoms in O3 positions from the upper and the bottom 6-mr(2Al)

of the hexagonal prism, respectively. Another configuration is also predicted, although energetically much less favorable (Table 3), in which Ni2þ is located in the plane of one of the two 6-mr (both identical in this cluster) at a SI0 6-mr(2Al) site, significantly shifted from the center of the ring (Figure 5b). Again, nickel at this position is close to the border of a high Al loading region and it binds two and one oxygen atoms located at O3 and O2 positions, respectively. For cluster B, only one configuration of 4-fold coordinated nickel is predicted (Table 3). In spite of the symmetry maintained in this configuration (in this cluster, Al loading is 5612



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Table 3. Ni2þCoordination and Distances (Ni-O and Ni-T) for the Preferable Nickel Locations in Ni/Na-Y (Si/Al = 2.4)a bond distances (Å)

cluster A

site


dNi-O3 (Å)

dNi-O2 (Å)

dNi-Al (Å)

dNi-Si (Å)

SINi/Na-Y(A)

3 (1.96 Å)

1.94

2.80

2.93

2.87

1.95

3.41

3.13

3.12

1.97

3.48

3.44

3.35

3.40

3.71

3.82

3.84

3.43

3.89

3.53

4.61


0.0

4.17 4.26 4.42

SI0 6-mr(2Al)

3 (1.91 Å)

1.86

2.00

2.75

4.47 2.66

1.86

3.42

3.40

3.29

3.07

3.60

2.10

3.27

3.36

3.19

2.13

3.40

3.50

3.20

2.18

3.50

3.52

3.36

2.24 3.04

3.71 3.73

3.78

3.43 3.44

3.09

4.09

þ8.3

3.90 3.99

cluster B

SINi/Na-Y(B)

4 (2.16 Å)

0.0

3.83 3.94 3.96

cluster C

SI0 6-mr(3Al)

4 (2.05 Å)

3 (1.96 Å)

1.94

2.15

2.94

2.81

1.98

3.37

3.42

3.34

2.12

3.43

3.47

3.37

1.91 1.96

2.87 3.17

3.18 3.27

3.06 3.21

1.99

3.27

3.34

3.24

0.0

þ6.2

a The coordination and related average distances (in brackets) are established from the dNi-O distances below 2.5 Å (in italic). ΔEconfig. refers to the relative stability of configuration regarding the most stable one for Ni/Na-Y.

Figure 6. Evolution of the Ni-O bond length average (Å) as a function of the Ni2þ coordination numbers, taken from the most stable optimized Ni/Na-FAU configurations (ΔEconfig. = 0 kcal/mol).

symmetrically distributed with respect to the center of the hexagonal prism), the Ni2þ cation is not stabilized in the center of the hexagonal prism but in a site, denoted SINi/Na-Y(B), that is located along the vertical axe (3-fold axis) and is shifted by 0.48 Å from the center of the prism (Figure 5c). There, Ni2þ is coordinated to four framework oxygen atoms all located at O3 positions with distinct Ni-O distances (Table 3). Finally, in the

case of cluster C, the nickel cation is stabilized exclusively in the plane of the 6-mr(3Al) of the prism, which is Al-richer than the opposite 6-mr(1Al) plane. Ni2þ coordination is then either 4-fold (Figure 5d) or 3-fold (Figure 5e) coordinated. The former configuration, where Ni2þ interacts with three and one oxygen atoms of the 6-mr(3Al) in O3 and O2 positions, respectively, is energetically the preferable one (Table 3). 3.2. Nickel-Oxygen Distances and Related Coordinations. The above detailed descriptions of Ni2þ sittings demonstrate that Ni2þ location and coordination significantly vary depending on the local framework Al content and distribution. By considering the most stable predicted configurations in each modeled system, a linear evolution of dNi-O average bond distances as a function of the Ni2þ coordination numbers can be drawn (Figure 6). This correlation explicitly shows that a lower framework Al content induces the simultaneous decrease of the nickel coordination number and global shortening of the Ni-O bonds; both parameters therefore appear strongly dependent. This general trend can also be analyzed in view of the experimental data available from literature. The average Ni-O bond length of 2.29 Å, predicted for nickel in site SI of Na-LSX (Table 1) is in good agreement with values found earlier by X-ray diffraction (XRD) for isolated nickel exchanged in dehydrated sodium faujasite. Thus, Gallezot et al.41 have measured Ni-O 5613



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Table 4. Energies of Exchange Reactions (Eexch) and Framework Deformations (Edeform) Calculated for the Energetically Most Stable Ni2þ Configurations in Each of the LSX, X, and Y Systems (i.e., Reference Levels ΔEconfig = 0 kcal mol-1)a site

Eexch (kcal.mol-1)

Edeform (kcal mol-1)

Ni/Na-LSX

SI

6 (2.29 Å)

-117.3

44.7

Ni/Na-X

SINi/Na-X SI0 6-mr(3Al)

5 (2.23 Å) 4 (2.05 Å)

-116.0 -116.3

43.7 48.7

Ni/NaY

a

coordination

cluster A

SINi/Na-Y(A)

3 (1.96 Å)

-114.8

39.2

cluster B

SINi/Na-Y(B)

4 (2.16 Å)

-97.5

50.7

cluster C

SI0 6-mr(3Al)

4 (2.05 Å)

-107.7

50.4

Site types and nickel coordinations with dNi-O average distances are recalled.

bond lengths in the range 2.29-2.37 Å and have assigned these distances to Ni2þ cations located in SI sites with a perfectoctahedral coordination. Using single-crystal X-ray techniques, D. Olson31 has similarly concluded that the Ni2þ ions occupy SI sites and attain near-perfect octahedral coordination, each being surrounded by six framework oxygen atoms at a distance of 2.29 Å. More recently, Seff and co-workers43 have also attributed Ni-O distances falling in the range 2.22-2.60 Å to octahedral Ni2þ located in site SI. This specific nickel coordination (i.e octahedral) indicates that nickel tends to adopt the highest possible coordination, in order to ensure charge compensation homogeneously distributed in the hexagonal prism. In the dehydrated faujasite series, only the LSX-like environment of the overall hexagonal prism can offer such a perfect octahedral coordination to nickel. Upon removing one framework Al atom among the 6 possible ones in the hexagonal prism (X faujasite), a competition between the 5-fold and 4-fold coordinations appears and dNi-O distances simultaneously decrease (Figure 6). Considering the Boltzmann energy distribution calculated at 298 K (Pi/Pj = exp(-ΔEij/RT) where ΣPi = 1), the relative proportions of nickel location inside the hexagonal prisms are found to be 1.4%, 26.6%, and 71.9% in sites SI0 6-mr(2Al), SINi/NaX, and SI0 6-mr(3Al), respectively. Applying this distribution, we find a Ni-O average bond length of 2.10 Å which is in fair agreement with the experimental Ni-O distances of 2.10 - 2.11 Å reported earlier for Na-X .13,36,40 In fact, using in situ dispersive-EXAFS, J. F. Groust and co-workers36 have followed the evolution of the Ni2þ environment in faujasite Na-X upon rehydration, and have shown that the best fit of the O shell (oxygen nearest neighbors of nickel) in the dehydrated sample is obtained for a Ni-O bond distance of 2.11 Å and a coordination number of about 5.5. Another agreement between this EXAFS study and the present theoretical prediction concerns the Ni-T experimental distances, found by EXAFS to be less than 3 Å, which concords very well with the present dNi-Si and dNi-Al distances calculated for Ni2þ located in SI0 sites in Ni/Na-X (2.84-2.92 Å, Table 2). The further decrease of the Ni2þ coordination numbers and of the dNi-O distances when decreasing the Al content in the hexagonal prism, i.e. in the Ni/Na-Y series, is easily explained by the fact that nickel, which is known for its strong affinity for the basic oxygen atoms of AlO4 tetrahedra, prefers to move toward Al-rich regions. As a consequence, nickel decreases its number of coordinated framework oxygens and it simultaneously gets closer to them, in positions that are either in the plane of the 6-mr or in a SI-type site (shifted from the center toward the wall of the prism). Actually, the Ni2þ coordination in Ni/Na-Y faujasites has been extensively studied using experimental

Figure 7. Schematic illustration of the framework deformation (before and after nickel introduction) in (a) LSX faujasite in which nickel is 6-fold coordinated in SI site and (b) in Y faujasite where nickel is 4-fold coordinated in SINi/Na-Y(B) site. Values indicate the oxygen framework shift and are reported in angstroms.

techniques such as powder XRD,38,40,41,43 diffuse reflectance spectroscopy (DRS),34,42 or EXAFS,37,38,75,76 but results are still subject of discrepancies (see for instance the work of Dooryhee et al.38). The EXAFS technique has the advantage to detect the individual and instantaneous local structure around the nickel ion, and it interestingly leads to Ni2þ environments that are in good concordance with our predicted configurations. Thus, the DFT calculated tetragonal coordination of nickel located in SI0 6-mr(3Al) (Table 3, Figure 5d) with dNi-O average bond length of 2.05 Å fits well with the nickel configuration reported by Sano et al.37 who have concluded to an average of 3.6 oxygen atoms in the vicinity of nickel and to dNi-O bond lengths of 2.05 Å. Similarly, a Ni-O bond length of 2.02 Å and a coordination number of (4 have been reported.38 Besides, the presence of a shorter nickelframework oxygen distance of 1.94 Å was mentioned, and explained by considering a configuration where Ni2þ is attached on the cage walls in trigonal coordination geometry, close to the negatively charged AlO4 units. This observed trigonal symmetry of nickel in Y zeolite, also detected by UV-visible spectroscopy by Lepetit et al.,42 highly agrees with the proposed SINi/Na-Y(A) DFT configuration (Table 3 and Figure 5a). Using synchrotron X-radiation and energy dispersive X-ray techniques, Seff et al.43 have reported three-coordinated near plane Ni2þ at site SI0 in dehydrated zeolite Y. This nickel is characterized by a short which is on the order nickel-framework oxygen distance of 1.99 Å of the DFT predicted one (1.96 Å) for 3-fold coordinated nickel in Ni-NaY cluster C (Table 3, Figure 5e). Finally some reported four-coordinated Ni2þ species in distorted tetrahedral symmetry (see ref 42) could correspond to a configuration equivalent to the 5614



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Table 5. Nickel-Oxygen Bond Lengths and Corresponding Mulliken Charges of Oxygen Framework in the 6-mr(3Al) of Ni/NaY (Cluster A)a before Ni introduction atom

dNa-O (Å)

Mulliken charge

3-fold coordinated Ni dNi-O (Å)

Mulliken charge

4-fold coordinated Ni dNi-O (Å)

Mulliken charge

O3

2.28

-0.78

1.91

-0.91

1.94

-0.88

O3

2.32

-0.77

1.96

-0.91

1.98

-0.88

O3 O2

2.30 2.79

-0.78 -0.77

1.99 2.87

-0.91 -0.76

2.12 2.15

-0.88 -0.86

O2

3.03

-0.76

3.17

-0.74

3.37

-0.74

O2

3.16

-0.76

3.27

-0.74

3.43

-0.73

Ni

1.28

1.29

Ni [core] 4s(0.31)3d(8.14)4p(0.01)

Ni [core] 4s(0.31)3d(8.17)4p(0.01)

a

Charges and Ni electronic configuration were derived from NBO analysis. The Mulliken charges of considered oxygen atoms in NaY (cluster C) (before Ni introduction) are also reported.

SINi/Na-Y(B) one (Table 3, Figure 5c). The varying nickel locations and coordinations described above can likely occur in Y since the Si/Al ratio (2 in the present models) implies that the AlO4 tetrahedra are spaced out in a less regular distribution (represented by the three anisotropic type configurations A, B, and C) than in X and LSX, the number of charges to be compensated for being moreover smaller. 3.3. Nickel-framework interaction. A complementary knowledge allowing comparison between the different nickel configurations in the considered faujasite systems is the strength of interaction between the transition metal ion and the framework. In order to evaluate this parameter, we considered a simplified exchange process approach, inspired from the one reported recently in a study on Pd/MOR.77 We thus calculated an energy for Ni2þ exchange in the host parent Na-FAU according to the reaction scheme Na - FAU þ ðNiðH2 OÞ6 Þ2þ þ 6H2 O f Ni=Na - FAU þ 2ðNaðH2 OÞ6 Þþ in which (i) the initial state consists in the gas phase microsolvated nickel cation, the dehydrated host sodium faujasite and the gas phase water molecules (needed to equilibrate the reaction) and (ii) the final state is made of the dehydrated nickel-exchanged faujasite and of the gas phase microsolvated sodium cations. The energy of this exchange reaction, Eexch, is estimated according to the equation: Eexch ¼ E½Ni=Na - FAU þ 2E½ðNaðH2 OÞ6 Þþ - E½ðNiðH2 OÞ6 Þ2þ - E½Na - FAU - 6E½H2 O This artificial chemical reaction, representing the binding strength between Ni2þ and the zeolite lattice, leads for all optimized Ni configurations to large negative exchange energies, in line with the feasibility of the exchange process. The Eexch values calculated for the energetically most stable configurations (Table 4) can be used, although with some caution, to compare the nickel interaction in the different zeolite environments. A general trend to be noted is the increase of Eexch (absolute values) as the Al content increases. This reflects the stronger Niframework interaction when more Al (more basic oxygen) atoms are present in the lattice. Thus, the strength of interaction is the highest for octahedrally coordinated Ni2þ inside the LSX hexagonal prism, and it slightly decreases for Ni2þ in Ni/Na-X.

Interestingly, both 5-fold and 4-fold coordinations adopted in Ni/Na-X lead to comparable Eexch values, which illustrates again the energetic equivalency of the SINi/Na-X and SI0 6-mr(3Al) sites in this system (same relative stability, Table 3); this further highlights the competition between the coordination and bond distances effects. In Ni/Na-Y, the nickel-framework interaction strongly depends on the cluster type (i.e., Al distribution) and it is found to increase as the dNi-O distance decreases. In cluster A that contains an Al-rich region along the wall, nickel adopts a 3-fold coordination in SINi/Na-Y(A) site, which leads to a high interaction with the hexagonal prism through short Ni-O bond lengths. In contrast, by maximizing its coordination with oxygen atoms, the 4-fold coordinated nickel interacts more weakly with the framework. The particularly small Eexch value calculated for SINi/Na-Y(B) can be easily explained by the much longer Ni-O distances imposed by the high Al dispersion in cluster B (Al atoms are positioned two-by-two on both sides of the hexagonal prism, far away from each other, Figure 5c). 3.4. Framework Deformations. IR analyses of various zeolite structures often reveal local framework deformation after the introduction of divalent TM cations.18,78,79 In a simple approach, one could expect a relationship between the strength of the TM interaction with the zeolite framework and the observed distortion. In order to evaluate such framework distortions in our models, we calculated the framework deformation energies for all energetically favorable nickel configurations, Edeform, as the difference between the energies (single point calculations) of the Ni/Na-FAU and Na-FAU cages, calculated after tacking of from the hexagonal prism the single nickel ion and the two sodium cations, respectively.

Edeform ¼ E½Ni=Na - FAU - Ni2- - E½Na - FAU - 2Na2From the obtained values, depicted in Table 4, no relationship between the deformation energy and the nickel-framework interaction strength can be established. For instance, although nickel located in SI site in Ni/Na-LSX has the highest interaction energy with the framework, it does not induce the highest framework deformation compared to the other Ni-faujasite systems. The highest deformation energies are found when nickel adopts the 4-fold coordination in X or Y, and they are related to both local (Ni2þ in SI0 6-mr(3Al)) and global (Ni2þ in SINi/Na-Y(B)) hexagonal prism distortions. 5615



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Figure 8. Oxygen charge distribution in the 6-mr(3Al) of Na-Y cluster C before (in presence of Na) and after nickel introduction. The 4-fold coordinated nickel in the SI0 6-mr(3Al) type site of the Ni/Na-Y cluster C, induces stronger framework distortion compared to the 3-fold coordinated one.

Compared to the Ni-free Na-LSX cluster (which geometric parameters provided in the Supporting Information), the distortion in Ni/Na-LSX is characterized by a shift of all of the six oxygen atoms of the hexagonal prism (O3 positions) by about 0.47 Å toward the nickel ion located at the center of the hexagonal prism (see Figure 7a). This DFT predicted result agrees well with the earlier IR observation of D. Olson.31 In contrast, the oxygen shifts induced by the 4-fold coordinated nickel in site SINi/Na-Y(B) (Ni/Na-Y cluster B, figure 7b) are less homogeneous. Moreover, relative to their positions in Nifree Na-Y cluster B, high displacements (0.68-0.43 Å) of the four O3 oxygen atoms bound to the nickel are found, which may explain the high deformation energy of 50.7 kcal/mol (Table 4). Interestingly, this important cage distortion occurs in spite of a weak nickel framework interaction (-97.5 kcal/mol) and of long ). These contrasting effects, that take Ni-O distances (2.16 Å place in a poor and heterogeneously dispersed Al environment, are related to the tendency of Ni2þ to increase its coordination number and can be summarized as follows: upon its search for an additional coordinating oxygen atoms, the nickel ion is forced to go away from the walls (longer average Ni-O bond, lower interaction) and it simultaneously pulls the coordinated oxygen atoms toward its direction (higher local framework distortion). The final configuration then corresponds to a compromise between these different balancing effects. Additional understanding of the parameters governing the stability of a given adopted Ni2þ coordination can be reached by analyzing the basicity strength of the neighboring framework oxygens. For this, let us analyze the oxygen charges in the two optimized configurations of tetragonal and trigonal nickel located in Ni/Na-Y cluster C (Table 5 and Figure 8). We chose these configurations because they well illustrate, for the same Al local environment, the competition between the Ni2þ propensity to either maximize its coordination with basic oxygens or to come close to them (see Table 3). Thus, in one case (4-fold coordination) the higher dNi-O distances are accompanied by a low interaction and strong deformation energies (Eexch = -107.7 kcal/mol and Edeform = 50.4 kcal/mol, Table 4) whereas in the other case (3-fold coordination), the short dNi-O distances induce nickel-framework interaction strengthening and framework deformation lowering (Eexch = -112.8 kcal/mol and Edeform = 43.2 kcal/mol, not reported in table 4). The Mulliken charges evaluated for these two systems (Table 5 and Figure 8) bring interesting features concerning the oxygen framework basicity and the preferred symmetry of nickel. First, it can be seen that the oxygen charges, which are almost all equivalent in Ni-free Na-Y, are no longer the same in

presence of nickel. Second, a relationship is found between the Ni-O bond distances and the framework oxygen charges, the tendency being an increase of the charge as the dNi-O bond length decreases. Finally, although the trigonal symmetry offers to nickel three short interactions with highly charged oxygen atoms (ΔEconfig = þ6 kcal/mol, Table 3), Ni2þ prefers to bind four oxygen atoms—although less charged—in order to maximize its coordination. In fact, an additional oxygen atom at O2 position is then involved in the Ni2þ coordination sphere, which increases the average dNi-O distance and the local deformation of the framework, Thus, relative to the oxygen charges before nickel introduction, the stabilization of Ni2þ ion within a 4-foldcoordination is accompanied by a more homogeneous charge distribution which concerns the maximum number of oxygens.

5. CONCLUSION The hosting of a divalent transition metal cation (Ni2þ) inside the hexagonal prism of sodium faujasites of different Si/Al ratios has been investigated by mean of DFT calculations. The aim was to clarify the precise sitting and coordination of Ni2þ within the prism that is known as the preferable nickel location in dehydrated Ni/Na-faujasite. Accurate structural information about the Ni2þ coordination and framework distortions are obtained. The consideration of several Al distributions in the modeled Na-LSX, Na-X, and Na-Y host clusters allows a thorough investigation of the influence of this parameter on the coordination and sitting of the transition metal ion. This work emphasizes the importance of considering different possible aluminum distributions, as well as a cluster model big enough, to reach a reliable description of framework deformation. The main insights on Ni location and Ni interaction with the basic framework oxygen atoms bared by the AlO4 tetrahedra can be summarized as follows: (i) The location and coordination of nickel is mainly influenced by the local anisotropy of the considered cages. Thus, a perfect octahedral coordination of Ni2þ in SI site can be reached only in the case of the highly isotropic LSX-like environment. Introducing a local anisotropy in the framework by considering the coexistence of Al-rich and Al-poor regions induces a decrease in the Ni coordination number. Thereby, the most probable coordinations which can be observed for nickel inside hexagonal prism faujasites with X- and Y-like environments are the 4-fold and 3-fold coordinations. (ii) The decrease of the Ni coordination number is accompanied by a decrease of the Ni-O bond lengths: 2.29 Å for 5616


The Journal of Physical Chemistry C 6-fold coordination, 2.23 Å for the 5-fold coordination, 2.04-2.16 Å for 4-fold coordination, and 1.91-1.96 Å for the 3-fold coordination. The symmetries adopted by the nickel are governed by the competition between its tendencies to either maximize its coordination with strong basic oxygens or to come close to them. (iii) The introduction of Ni2þ inside the hexagonal prism causes local distortions of the zeolite framework which mainly depend on the AlO4 distribution. The stronger framework deformation is found for faujasite-Y where, by adopting the 4-fold coordination, the nickel maximizes its interaction with basic oxygen atoms. A high framework distortion cannot be systematically assigned to a high nickel-framework interaction.

’ ASSOCIATED CONTENT

bS

Supporting Information. DFT optimization of the host Na-LSX, Na-X and Na-Y clusters. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Telephone: þ33 (0)1 44 27 36 26. Fax: þ33 (0)1 44 27 60 33. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was granted access to the HPC resources of [CCRT/ CINES/IDRIS] under the allocation 2009 [x2009082022] made by GENCI (Grand Equipement National de Calcul Intensif]. Lucia Gaberova and Frederik Tielens are warmly thanked for their interest and fruitful discussions. ’ REFERENCES (1) Kulkarni, S. J.; Madhavi, G.; Rao, A. R; Mohan, K. V. Catal. Commun. 2007, 9, 532. (2) Madhavi, G.; Kulkarni, S. J.; Raghavan, K. V. J. Porous Mater. 2007, 14, 379. (3) Romero, M. D.; Ovejero, G.; Rodriguez, A.; Gomez, J. M.; Agueda, I. Indust. & Eng. Chem. Res. 2004, 43, 8194. (4) Corma, A.; Garcia, H.; Leyva, A.; Primo, A. Appl. Catal. A: General 2003, 247, 41. (5) Pinard, L.; Magnoux, P.; Ayrault, P.; Guisnet, M. J. Catal. 2004, 221, 662. (6) Beauchet, R.; Magnoux, P.; Mijoin, J. Catal. Today 2007, 124, 118. (7) Morris, R. E.; Wheatley, P. S. Angew. Chem. 2008, 47, 4966. (8) Li, Y.; Yang, R. T. J. Phys. Chem. B 2006, 110, 17175. (9) Barthomeuf, D. Catal. Rev. Sci. Eng. 1996, 38, 521. (10) De Mallmann, A.; Barthomeuf, D. J. Chim. Phys. Phys.—Chim. Biol. 1990, 87, 535. (11) Bisio, C.; Fajerwerg, K.; Krafft, J.-M.; Massiani, P.; Martra, G. Res. Chem. Intermed. 2008, 34, 565. (12) Becue, T.; Maldonado-Hodar, F. J.; Antunes, A. P.; Silva, J. M.; Ribeiro, M. F.; Massiani, P.; Kermarec, M. J. Catal. 1999, 181, 244. (13) Mielczarski, E.; Hong, S. B.; Davis, R. J.; Davis, M. E. J. Catal. 1992, 134, 359. (14) Pierloot, K.; Delabie, A.; Groothaert, M. H.; Schoonheydt, R. A. Phys. Chem. Chem. Phys. 2001, 3, 2174. (15) Berthomieu, D.; Delahay, G. Catal. Rev.—Sci. Eng. 2006, 48, 269. (16) Berthomieu, D.; Ducere, J.-M.; Goursot, A. J. Phys. Chem. B 2002, 106, 7483.

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