Isolated Chromium(VI) Oxide Species Supported ... - ACS Publications

Article pubs.acs.org/JPCC

Isolated Chromium(VI) Oxide Species Supported on Al-Modified Silica: A Molecular Description Jarosław Handzlik,*,† Robert Grybos,‡ and Frederik Tielens§ †

Faculty of Chemical Engineering and Technology, Cracow University of Technology, ul. Warszawska 24, 31-155 Kraków, Poland Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul. Niezapominajek 8, 30-239 Kraków, Poland § Sorbonne Universités, UPMC Univ Paris 06, UMR 7574, Laboratoire Chimie de la Matière Condensée, Collège de France, 11 place Marcelin Berthelot, 75231 Paris Cedex 05, France ‡

S Supporting Information *

ABSTRACT: Supported chromium oxide systems are widely used catalysts. Experimental data indicate that modification of the silica support with Al changes the properties of the surface Cr species. In this computational work, extensive density functional theory investigations of the monomeric Cr(VI) oxo species variously located on the Al-modified silica surface have been done using a large number of systematically generated periodic and cluster models. It is predicted that the surface modification can influence both the geometry and the relative stabilities of the Cr species. Monooxo Cr(VI) species located in the vicinity of two Al sites usually adopts a pseudotetrahedral geometry, in contrast to square pyramidal monooxo Cr(VI) species on silica. Under dehydrated conditions, the relative stabilities of the tetrahedral monooxo and dioxo Cr(VI) species are comparable, whereas the dioxo Cr(VI) species is thermodynamically preferred on silica. The obtained results explain the experimentally estimated higher ratio of the monooxo to dioxo surface Cr(VI) species for CrO3/ AlOx/SiO2, compared to the chromia−silica system. Calculated CrO stretching frequencies for the monooxo and dioxo Cr(VI) species on the Al-modified silica surface are in a range similar to those predicted for the corresponding Cr(VI) species on silica, in agreement with reported experimental data.

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INTRODUCTION Supported chromium oxide systems are efficient catalysts for many important chemical processes, such as ethene polymerization,1−8 dehydrogenation8,9 and oxidative dehydrogenation10−12 of alkanes, selective oxidation reactions,1,13,14 and methane dehydroaromatization.15 Highly dispersed surface Cr(VI) species, predominantly present on the catalyst surface after calcination, are the active sites or their precursors.1−15 In the case of the widely used chromia−silica system, they are most likely monomeric species. Experimental and theoretical investigations indicate the presence of tetrahedral dioxo Cr(VI) species on the silica surface under dehydrated conditions.1,3,5,16−21 Additionally, 5-coordinated monooxo Cr(VI) forms have been proposed as minor species.18−21 Modification of chromia−silica systems with Al can significantly influence their catalytic properties.4,6,7,14 In the case of multilayered supported CrO3/AlOx/SiO2 catalysts, Raman spectroscopy studies indicate the presence of both dioxo and monooxo Cr(VI) species on the surface (Figure 1).22,23 Interestingly, the estimated ratio of the monooxo to dioxo surface Cr(VI) species is two-fold higher for the CrO3/ AlOx/SiO2 system (1:1), compared to the CrO3/SiO2 catalyst (1:2), which can be one of the possible reasons for their different catalytic activities. © 2016 American Chemical Society

Figure 1. Proposed structures for the monomeric Cr(VI) oxide species on AlOx/SiO2.23

Reported computational studies of supported chromium oxide were mainly limited to chromia−silica systems,16,21,24−30 and very few papers concern the Cr/SiO2 catalyst modified with Al7 or chromium oxide species in zeolite.15,31 In most cases, rather small and/or arbitrary proposed cluster models of Cr-containing systems were used.7,16,25−30 While small models enable effective exploration of reaction mechanisms,7,27−30 they cannot fully reproduce the complexity of the surface and heterogeneity of the metal sites. Recently, an advanced periodic model of amorphous silica32 was adapted to study surface chromium(VI) oxo species under partially hydrated24 and Received: June 6, 2016 Revised: July 11, 2016 Published: July 12, 2016 17594

DOI: 10.1021/acs.jpcc.6b05675 J. Phys. Chem. C 2016, 120, 17594−17603

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The Journal of Physical Chemistry C dehydrated21 conditions. In parallel, large cluster models, representing amorphous silica, were also applied.21 Modeling amorphous silica−alumina materials is an even more challenging task. In the past, silica−alumina was represented by cluster models limited to 1−2 Al or Si atoms.33 Such models were also recently used to calculate reaction pathways for ethene polymerization on the Almodified Phillips catalyst.7 A modified silsesquioxane cube was proposed as a model for the surface of silica−alumina to study supported rhenium oxo species.34,35 By simulating deposition of a silica film on γ-alumina surface, advanced periodic models of amorphous aluminosilicate surface were developed enabling detailed description of Brønsted acid sites and their adsorption properties and reactivity.36−42 Recently, grafting of trialkylaluminum on silica was computationally studied, based on the β-cristobalite structure43 or a periodic model of amorphous silica.44 Previously, comprehensive density functional theory (DFT) investigations of the CrO3/SiO2 system were performed.21,24 It was shown that surface dioxo Cr(VI) species are more stable than the monooxo Cr(VI) species under dehydrated conditions, in accordance with their experimentally estimated ratio.20,22,23 This thermodynamic preference is stronger than for the analogous molybdena−silica system.45 In both cases, the calculated relative energies of the surface metal species depend on the local silica structure, indicating the importance of developing realistic models that are able to represent the heterogeneity of the surface metal sites. In the present work, we have undertaken a comprehensive computational study of the chromium oxide supported on Almodified silica to shed light on the nature of the surface chromium species and to understand the reason for the higher ratio of the monooxo to dioxo Cr(VI) species, compared to the CrO3/SiO2 system. The effect of dehydration of the chromium sites on their structure and relative stabilities is also examined. The Al-modified silica surface has been modeled by a modification of the well-established periodic model of amorphous silica,32 successfully employed in theoretical studies of Cr(VI),21,24 Mo(VI),46 W(VI),47 V(V),48 Nb(V),49 and Au50,51 oxide species on SiO2. In addition to periodic calculations, corresponding cluster models have been generated to estimate relative Gibbs energies of the surface chromium structures. This way, we use a variety of systematically developed periodic and large-cluster models, which enable us to sample the site heterogeneity of the amorphous catalyst surface. To the best of our knowledge, theoretical investigations of chromium oxide species supported on amorphous silica− alumina or AlOx/SiO2 materials have not been reported to date.

Density functional theory calculations have been performed with the Perdew and Wang (PW91) exchange−correlation functional,52 which was previously used for investigations of CrO3/SiO2 system21 and was selected on the basis of assessment of various DFT methods for thermochemistry of chromium oxo compounds.53 For valence electrons, a planewave basis set has been applied. Atomic cores are described with the projector-augmented wave method (PAW).54 Standard PAW atomic parameters are used, requiring a cutoff energy of 400 eV (fixed by the oxygen atom). For chromium, the PAW is built with 12 electrons in the valence. The Γ point is used in the Brillouin zone integration. To determine harmonic vibrational frequencies, the Hessian matrix has been computed by the finite difference method. The chromium atom and its neighbors, at least within the second coordination sphere, are included in the Hessian matrix. The presented vibrational frequencies are scaled by the previously determined scaling factor of 0.9241.21 The calculations are done with the Vienna ab Initio Simulation Package (VASP).55−57 For the graphic presentation of the structures, Materials Studio 6.0 software has been applied.58 Cluster Calculations. The main series of cluster models of monomeric Cr(VI) species on Al-modified silica, containing 31−32 Si atoms and 1−2 Al atoms, has been developed on the basis of the corresponding periodic models considered in this work. In another approach, one or two AlO(OH) units and Cr(VI) monomer have been deposited on an analogous silica cluster containing 33 Si atoms. One more family of cluster models, containing 21, 24, and 26 Si or Al atoms, have been derived adapting another structure of amorphous silica (Materials Studio database58), which was previously applied for investigations of surface chromium21 and molybdenum45 species. In all cases, there are one or two Al sites in the vicinity of the Cr(VI) center. The dangling bonds have been saturated with hydrogen atoms to form hydroxyl groups at the cluster periphery. For each system, full geometry optimization, including the terminated H atoms, has been carried out using the PW91 functional combined with the split-valence def2-SVP basis set.59 Vibrational frequencies and Gibbs free-energy corrections have been evaluated in the harmonic oscillator and rigid-rotor approximations. The presented vibrational frequencies are scaled by the scaling factor of 0.9149, determined previously.21 The Gibbs energy corrections have been added to PW91 singlepoint energies calculated with the triple-ζ valence def2-TZVPP basis set,59 to obtain a better estimation of Gibbs energies (ΔG). The reported energies (ΔE) are not zero-point-energycorrected to enable comparison with the results obtained from the periodic calculations. This methodology was previously successfully applied in computational studies of Cr/SiO221 and Cr/ZSM-515 systems. All calculations have been performed with the Gaussian 09 set of programs.60 Nomenclature. The nomenclature of the models is constructed by a number (1−3), a capital or small letter and, in most cases, a superscript roman numeral (I−IV). The numbers 1, 2, and 3 denote the monooxo, partially hydrated dioxo, and dehydrated dioxo Cr(VI) species, respectively (Figure 2). The capital letter describes a periodic model, whereas the small letter specifies a cluster model. The superscripts are used to distinguish among different locations of the chromium sites on the surface represented by the same model, or between analogous models which differ from each

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COMPUTATIONAL MODELS AND METHODS Periodic Calculations. The periodic models of monomeric chromium oxide species supported on Al-modified silica have been prepared on the basis of the amorphous silica structure,32 which was partially dehydroxylated21 and variously modified by replacing one or two surface Si atoms per unit cell by Al atoms (vide infra). In the latter case, Al−O−(Si−O)2−Al sequence was assumed, allowing the modeling of the Cr center with two Al sites in its direct vicinity. The surface unit cell dimensions are a = 12.77 Å, b = 17.64 Å, c = 25.17 Å, including 15 Å of vacuum. The chromium coverage in the models is about 0.4 atoms nm−2, i.e., one Cr atom per unit cell. The positions of all atoms in the unit cell have been relaxed during geometry optimization. 17595


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The Journal of Physical Chemistry C

Figure 2. General view of the monooxo (1) and dioxo (2, 3) Cr(VI) sites studied in this work.

other by the location of the proton compensating the framework charge. The periodic and cluster models corresponding to each other are denoted analogously, for instance 1AI and 1aI.

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RESULTS AND DISCUSSION Structure and Relative Energies of Surface Cr Species. To examine how the modification of the silica surface with Al can influence the structure and stability of the chromium species, the previously reported21 periodic model of the monooxo Cr(VI) species on dehydrated silica has been used as the starting structure. At first, one Al site has been placed in the direct vicinity of the Cr center, by replacing Si atom in the amorphous structure of the support. In models 1AI and 1BI, two different locations of the Al site are considered (Figure 3). In both structures, a Cr−O−Al bridge is formed, but the geometry of the monooxo Cr(VI) species is not dramatically changed, in comparison to the corresponding species located on pure silica.21 They are still 4-fold bonded to the surface and constitute a distorted square pyramid (Figure 2). On the basis of these periodic systems, analogous cluster models (1aI and 1bI; Figure 3) have been prepared. The whole cluster structure for the model 1aI is shown in Figure S1 (Supporting Information). Various possible locations for the additional proton that compensates the framework charge forming Si− (OH)−Al Brønsted acid site are also taken into account (see models 1AII, 1AIII, 1aII, 1aIII, 1BII, 1BIII, 1bII, and 1bIII; Figure S1). The relative energies of these systems are more or less different (Table S1). On the basis of the Gibbs energies (T = 873 K), 1aI and 1bI are most stable among the cluster models, with 1bI being more thermodynamically preferred to 1aI, by 16 kJ·mol−1. This preference is larger in terms of electronic energies (T = 0 K), especially for the corresponding periodic models. Considering the Si−(OH)−Al Brønsted acid sites represented by the periodic models, the calculated Al−O distances (1.84−2.28 Å) are always higher than the Si−O bond lengths (1.72−1.82 Å). For the most stable monooxo Cr(VI) species (1AI, 1BI), the Al−O distance is about 2 Å, whereas for the less stable species (1BIII), the Al−O and Si−O bonds in the Si− (OH)−Al site are almost equivalent to each other (1.84 and 1.82 Å, respectively). The highest O−Al distance (2.28 Å) is obtained for model 1BII (Figure S1), and this case might be classified as a pseudobridging silanol.36−42 On the other hand, more flexible cluster models usually predict shorter Al−O bonds for the Brønsted acid sites (1.86−1.88 Å), with the exception of 1bIII (1.96 Å) and 1aII (2.03 Å). The latter represents a 5-coordinated Al site, while in other periodic and cluster models, pseudotetrahedral Al sites are predicted. In

Figure 3. Optimized structures of the monooxo (1AI, 1aI, 1BI, 1bI) and dioxo (2BI, 2bI, 3BI, 3bI) Cr(VI) species on Al-modified silica, represented by the periodic (1AI, 1BI−3BI) and corresponding cluster (1aI, 1bI−3bI) models. One Al site per Cr site is assumed.

many cases, the acidic proton interacts with an oxygen atom from a silanol group, Si−O−Si bridge, or Cr−O−Si bridge, with the O···H distance of 1.80 Å (periodic models) or 1.47− 1.59 Å (cluster models). The periodic and cluster models of variously located dioxo Cr(VI) species have been constructed from the most stable monooxo Cr(VI) structures 1BI and 1bI by hydrolysis of two adjacent Cr−O−Si or Cr−O−Al linkages (2BI−2BIV, 2bI− 2bIV) or by direct rearrangement (3BI−3BIII, 3bI−3bIV). Selected representatives are shown in Figure 3, whereas the remaining structures are presented in Figure S1. Cluster models of the dioxo species generated from the monooxo model 1aI have also been calculated (Figure S1 and Table S1), but they will not be discussed in detail. Similar to the CrO3/SiO2 system,21 all these dioxo Cr(VI) species are pseudotetrahedrally coordinated. In some cases (2BIII, 3BI, 3BIII), the Si−(OH)−Al site is no longer present, being decomposed into 3-coordinated Al center and a silanol group. In the remaining periodic models, the predicted Al−O distances for the Brønsted acid sites are longer (2.05−2.24 Å) than in the monooxo Cr(VI) site 1BI. A similar trend is observed for the corresponding cluster models. Many models (2BIII, 2BIV, 3BI, 3BIII, 2bI, 2bII, 2bIV, and 3bI− 3bIII) predict interactions between the oxo ligands and neighboring silanols (the O···H distance is 1.83−2.48 Å), analogous to dioxo Cr(VI) species on silica.21 17596


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Table 1. Energies (ΔE, kJ·mol−1) and Gibbs Energies at T = 873 K (ΔG873, kJ·mol−1) for the Conversion of the Dioxo Cr(VI) Species (2BI, 2CI, 3BI−3DI, 2bI, 2cI, 2eI, 2fII, 2p, 3bI−3fI, 3g, 3p) to the Monooxo Species (1BI, 1C, 1DI, 1bI, 1c, 1dI−1fI, 1g, 1p)

a

ΔE

Al/Cra

reactionb

1 1 1 2 2 2 1 1 2 2 2 2 2

2B → 1B + H2O 3BI → 1BI

152 112

2CI → 1C + H2O 3CI → 1C 3DI → 1DI

133 103 −114

I

I

reactionc

ΔE

ΔG873

2b → 1bI + H2O 3bI → 1bI 3bIV → 1bI 2cI → 1c + H2O 3cI → 1c 3dI → 1dI 2eI → 1eI + H2O 3eI → 1eI 2fII → 1fI + H2O 3fI → 1fI 3g → 1g 2p → 1p + H2O 3p → 1p

118 66 198 136 89 2 172 103 116 −28 9 22 −35

8 61 116 3 98 44 21 58 3 −37 38 −94 −8

I

Number of Al sites per Cr site in the model. bPeriodic models. cCluster models.

Table 2. Gibbs Energies (kJ·mol−1) for the Dehydration of the Dioxo Cr(VI) Species (2bI, 2cI, 2eI, 2fII, 2p) to the Monooxo Species (1bI, 1c, 1eI, 1fI, 1p) as a Function of Temperature and Water Vapor Pressure 473 K a

a

b

Al/Cr

reaction

1 2 1 2 2

2b → 1b + H2O 2cI → 1c + H2O 2eI → 1eI + H2O 2fII → 1fI + H2O 2p → 1p + H2O I

I

10

−2

atm

673 K 10

37 43 68 35 −62

−5

atm

10 16 41 7 −89

10

−2

atm

5 6 28 2 −95

873 K −5

10

atm

−33 −33 −11 −37 −134

−2

10

atm

−25 −31 −12 −31 −128

10−5 atm −75 −81 −62 −81 −178

Number of Al sites per Cr site in the model. bCluster models.

A variety of models representing Cr(VI) species located in the vicinity of two Al sites are presented in Figure 4 and Figure

Among the dehydrated dioxo Cr(VI) species represented by the periodic models, 3BI is most stable (Table S1), because of easy dehydroxylation of 2BI, involving two adjacent silanols. In the case of 3BII, a strained two-member (Si, Al) ring is formed after the dehydration of 2BII. Even more significant surface reconstruction occurs during the dehydration of 2BIII to 3BIII, resulting in a new oxygen bridge between initially remote Si and Al atoms. For the corresponding cluster models, the energy also increases from 3bI to 3bIII, but 3bIV is the most stable system. In the case of the more rigid periodic models, dehydration of 2BIV leads to the monooxo Cr(VI) species 1BI. To compare the relative stabilities of the most stable monooxo and dioxo Cr(VI) species, the energetic effects for the dehydration reactions of the dioxo Cr(VI) species (2BI, 2bI) to the corresponding monooxo species (1BI, 1bI) and for the direct transformation of the dehydrated dioxo species (3BI, 3bI, 3bIV) to the monooxo species have been calculated (Table 1). The dehydration reactions are endoenergetic for all periodic and cluster models; however, at high temperatures and under low water vapor pressure, the formation of the monooxo species becomes preferable (Table 2, the results for 1bI). On the other hand, alternative dehydration reactions resulting in formation of the most stable dioxo species are even more preferred. Consequently, these species (3BI, 3bI, 3bIV) are energetically more favorable than the corresponding monooxo structures (Table 1). Similar conclusions can be drawn from the results obtained from series a of the cluster models (Tables S2 and S3). In the case of the silica support, the dehydrated dioxo Cr(VI) species are also thermodynamically preferred over the monooxo Cr(VI) species.21 The present results show that this preference is not significantly influenced by introducing to the silica support one Al site in the neighborhood of the Cr center.

Figure 4. Optimized structures of the monooxo (1CI, 1c) and dioxo (2CI, 2cI, 3CI, 3cI) Cr(VI) species on Al-modified silica, represented by the periodic (1C, 2CI, 3CI) and corresponding cluster (1c, 2cI, 3cI) models. Two Al sites per Cr site are assumed. 17597


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The Journal of Physical Chemistry C S2. Their relative energies are compared in Table S4. The monooxo species (1CI, 1cI) still show square pyramidal geometry, whereas the dioxo species (2CI−2CIV, 2cI−2cIV, 3CI−3CIV, 3cI−3cIV) are pseudotetrahedrally coordinated. The models represent different locations of the dioxo species. Additionally, several possible variants of dehydration are considered for 3CIII (3CIIIa−3CIIId) and 3cIII (3cIIIa−3cIIId). When the most stable systems are regarded (Figure 4), there are two Brønsted acid sites in the vicinity of each Cr(VI) center (the Al−O distance is 1.97−2.07 Å and 1.86−1.93 Å for the periodic and cluster models, respectively), with the exception of 3CI, where one Si−(OH)−Al site is decomposed into 3coordinated Al center and a silanol group. Again, for many dioxo species (2cI, 3CI, 3cI), interactions between oxo ligands and neighboring silanols are predicted (the O···H distance is 1.60−1.77 Å). The energies (T = 0 K) and Gibbs energies for the reactions transforming dioxo species (2CI, 3CI, 2cI, 3cI) to the monooxo species (1C, 1c) are given in Tables 1 and 2. Similarly to the models with one Al site per Cr species and to the Cr(VI)/SiO2 system,21 these results indicate that the dehydrated dioxo Cr(VI) species are more stable than the monooxo Cr(VI) species. Hence, even introducing two Al sites near the Cr(VI) center does not change the thermodynamic preferences if the geometry of the species is preserved. The presence of two Brønsted acid sites close to Cr site enables further dehydration by removing both protons and one bridge oxygen ion from a Cr−O−Al bond, accompanied by respective surface reconstruction and resulting in new monooxo (1DI−1DIII, 1dI−1dIII) and dioxo (3DI−3DIII, 3dI, 3dIa, 3dIII) Cr(VI) species (Figure 5). Their relative energies are given in Table S4. It is clear that almost all monooxo species are now 3fold bonded to the surface and exhibit pseudotetradedral geometry (Figure 2). Only in the case of the cluster model 1dI is the square pyramidal coordination still preserved. Most dioxo species are pseudotetrahedrally coordinated, with exception of the cluster models 3dI and 3dIa representing unusual 5-fold coordination of the Cr centers. Each of these Cr atoms is bonded to the surface via Cr−O−Al bridge and additionally interacts with one Si−O−Al and one Si−O−Si bridge oxygen (the Cr−O distance is 2.06−2.19 Å). Bonding between the Cr atom and the Si−O−Al bridge oxygen is also predicted for the tetrahedral dioxo species 3DI and 3DII (1.90−1.92 Å; periodic models), as well as for the square pyramidal 1dI and tetrahedral 1dIII monooxo species (1.85−2.12 Å; cluster models). Except pseudotetrahedral Al sites, 3-coordinated Al sites are also present in some models (1DI, 1DIII, 3DIII, 1dII, and 3dIII). The predicted relative stabilities of the monooxo and dioxo Cr(VI) species in the vicinity of two Al sites change significantly after dehydration (Table 1). This is especially seen for the tetrahedral monooxo Cr(VI) species 1DI, which is clearly more stable than the corresponding dioxo species 3DI (ΔE = −114 kJ·mol−1), in contrast to the less dehydrated models 1C and 3CI and to the analogous Cr(VI) species on silica (ΔE = 69 kJ·mol−1).21 Although the cluster calculations still indicate that the dioxo species (3d1) is thermodynamically favored over the monooxo one (1dI), this preference is much weaker than for the corresponding less dehydrated species (3cI, 1c) and for the analogous cluster models representing Cr(VI)/ SiO2 system.21 Moreover, both periodic and cluster calculations show that the dehydration is preferential in the case of the monooxo Cr(VI) species (Tables 3 and 4). Considering the dioxo species, the dehydrated system 3dI is predicted to be

Figure 5. Optimized structures of the monooxo (1DI−1DIII, 1dI− 1dIII) and dioxo (3DI−3DIII, 3dI−3dIII) Cr(VI) species on Almodified silica, represented by the periodic (1DI−1DIII, 3DI−3DIII) and corresponding cluster (1dI−1dIII, 3dI−3dIII) models. Two Al sites per Cr site are assumed.

more stable than 3cI under dehydrated conditions. Hence, the modification of the silica surface with Al allows for its deeper dehydration and can increase the relative stability of the monooxo Cr(VI) species, especially if they adopt tetrahedral geometry. Another family of the cluster models have been prepared by grafting one or two AlO(OH) units and Cr(VI) monomer on the silica surface (Figures 6 and 7 and Figure S3). Their relative energies are given in Tables S5 and S6. Series e and f of the models represent Cr(VI) species neighboring one and two Al sites, respectively. As previously discussed, Si−(OH)−Al Brønsted acid sites are reproduced by the models. The calculated Al−O bond lengths are usually in the range of 17598


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The Journal of Physical Chemistry C Table 3. Energies (ΔE, kJ·mol−1) and Gibbs Energies at T = 873 K (ΔG873, kJ·mol−1) for the Dehydration Reactions of the Monooxo (1C, 1c, 1fI) and Dioxo (3CI, 3cI, 3fI) Cr(VI) Species; Two Al Sites per Cr Site Are Assumed reactiona

ΔE

reactionb

ΔE

ΔG873

1C → 1D + H2O

−11

3CI → 3DI + H2O

206

1c → 1dI + H2O 1c → 1dII + H2O 3cI → 3dI + H2O 1fI → 1g + H2O 3fI → 3g + H2O 1fI → 3g + H2O

75 49 161 198 161 189

−59 −41 −5 100 25 62

I

a

Periodic models. bCluster models.

Table 4. Gibbs Energies (kJ·mol−1) for the Dehydration Reactions of the Monooxo (1c, 1fI) and Dioxo (3cI, 3fI) Cr(VI) Species as a Function of Temperature and Water Vapor Pressure; Two Al Sites per Cr Site Are Assumed 473 K −2

−2

873 K

reactiona

10 atm

10 atm

10 atm

10 atm

10 atm

10−5 atm

1c → 1d + H2O 1c → 1dII + H2O 3cI → 3dI + H2O 1fI → 1g + H2O 3fI → 3g + H2O 1fI → 3g + H2O

−18 −22 52 119 65 97

−45 −49 25 92 38 70

−55 −49 6 92 28 63

−94 −87 −32 53 −11 24

−92 −74 −39 66 −9 29

−143 −124 −89 16 −59 −22

I

a

673 K −5

−5

−2

Figure 7. Optimized structures of the monooxo (1fI, 1fII) and dioxo (2fI−2fIII, 3fI) Cr(VI) species on Al-modified silica, represented by cluster models. Two Al sites per Cr site are assumed (two AlO(OH) units deposited on the silica surface).

(series f) and in 3eI. The geometry of the monooxo Cr(VI) species can be square pyramidal (1eI) or, more often, tetrahedral (1eII, 1fI, 1fII). Similarly to other models, many 4coordinated dioxo Cr(VI) species interact with surface OH groups (the O···H distance is 1.49−1.94 Å). Among the Cr(VI) species with one Al site in the vicinity, the dioxo species 3eI is the most stable one under dehydrated conditions (Tables 1 and 2 and Table S5), in agreement with the analogous results obtained using series a and b of the models. This confirms that such modification of the silica surface does not change the thermodynamic preference for the dioxo Cr(VI) species, previously reported for chromia−silica system.21 However, if two Al sites are present near Cr, tetrahedrally coordinated monooxo species (1fI) are thermodynamically favorable under dehydrated conditions (Tables 1 and 2 and Table S6). Possible dehydration of Brønsted acid sites in 1fI and 3fI may lead to new tetrahedral monooxo (1g) and dioxo (3g) Cr(VI) species, respectively (Figure 8). Only the second reaction is predicted to be thermodynamically favorable under strict dehydrated conditions (Tables 3 and 4). It is also seen that 1g is less stable than 3g (Table 1). Hence, the thermodynamic equilibrium between the monooxo Cr(VI) species 1fI and more dehydrated dioxo species 3g can indicate which functionality is more probable on the surface. Very strict dehydrated conditions

Cluster models.

Figure 6. Optimized structures of the monooxo (1eI, 1eII) and dioxo (2eI, 3eI) Cr(VI) species on Al-modified silica, represented by cluster models. One Al site per Cr site is assumed (AlO(OH) unit deposited on the silica surface).

1.88−1.95 Å, but sometimes longer distances (2.00−2.06 Å) are predicted (1eII, 3eIV, 1fII, 3fI). In the model 1fI, representing a stable monooxo Cr(VI) species, a pseudobridging silanol36−42 is present (the Al−O distance is 2.37 Å). The most asymmetric Brønsted sites are often composed of 5coordinated Al (1eII, 1fI, 1fII). In other cases there are pseudotetrahedral Al sites, in agreement with the recent dynamic nuclear polarization surface-enhanced NMR spectroscopy studies of the Al-modified silica, indicating that the formation of Brønsted acid sites is related to the presence of 4coordinated Al.42 Interactions between the acidic proton and a silanol, siloxane, or CrO moiety (the O···H distance of 1.49− 2.13 Å) are observed in the models containing two Al sites

Figure 8. Optimized structures of the monooxo (1g) and dioxo (3g) Cr(VI) species on additionally dehydrated Al-modified silica, represented by cluster models. Two Al sites per Cr site are assumed (two AlO(OH) units deposited on the silica surface). 17599


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monooxo (1m) and dioxo (3m) Cr(VI) species are close to equilibrium, with the latter being slightly preferred (Table S9). Analogous to the models with one Al site, by removing one water molecule, stable systems are obtained under dehydrated conditions (series n). However, further dehydration (series p, Figure 9) is even more exergonic (Table S11). In the latter case, the Cr(VI) monooxo species (1p) is thermodynamically preferred over the dioxo species (Tables 1 and 2). It is worth emphasizing that in this set of models all monooxo Cr(VI) species neighboring two Al sites (1m, 1nI, 1nII, 1p) exhibit pseudotetrahedral geometry and they are 3-fold bonded to the surface (Figure 2). To summarize, the computational results obtained with a large number of periodic and cluster models representing surface Cr(VI) oxo species variously located on Al-modified silica indicate that the surface modification can influence both the geometry and the relative stabilities of the Cr species. This effect is seen when two Al sites are located in the vicinity of the Cr center. First, the monooxo Cr(VI) species usually adopt pseudotetrahedral geometry, in contrast to those located on pure silica, which have square pyramidal coordination. Second, clear thermodynamic preference for the dioxo Cr(VI) species, over the monooxo species, typical for Cr(VI)/SiO2 system, is no more predicted. Instead, the relative stabilities of both species are more or less comparable, depending on their location and the model applied. These findings can explain why the experimentally observed ratio of the monooxo to dioxo surface Cr(VI) species is higher for the CrO3/AlOx/SiO2 system (1:1), compared to the CrO3/SiO2 catalyst (1:2).22,23 Vibrational Frequencies. Theoretically determined Cr O stretching frequencies for the monooxo and dioxo Cr(VI) species are presented in Table 5 and are compared with the corresponding Raman spectroscopy data.22,23 The computed vibrational modes are coupled with the support vibrations, similar to the previously studied chromia−silica system.21 The experimental CrO stretching frequencies reported by Lee and Wachs22,23 for the CrO3/AlOx/SiO2 system under dehydrated conditions and assigned to the surface monooxo and dioxo Cr(VI) species are practically the same as those measured for the CrO3/SiO2 system.18,19 In agreement with these Raman data, the calculated CrO vibrational frequencies for the Cr(VI) species on Al-modified silica are approximately in the same ranges as those predicted for the chromia−silica catalyst.21

favor the dioxo species; otherwise, the monooxo species is more stable (Tables 3 and 4). However, at lower temperatures and higher water vapor pressure, hydrated dioxo species (2fII) become thermodynamically preferred over the monooxo species (1fI) (Table 2). Hence, these results indicate that both tetrahedral monooxo and dioxo Cr(VI) species can be formed in the vicinity of two Al sites on the AlOx/SiO2 surface and that their relative stabilities depend on the conditions. One more set of cluster models (Figures S4 and S5; Figure 9) has been derived on the basis of another structure of

Figure 9. Optimized structures of the monooxo (1p) and dioxo (2p, 3p) Cr(VI) species on dehydrated Al-modified silica, represented by cluster models. Two Al sites per Cr site are assumed.

amorphous silica,58 previously also applied for modeling surface chromium species.21 Series h, i, j, and k of the models represent variously located surface Cr(VI) species having a lone Al site in the neighborhood. In accordance with other results, clear thermodynamic preference for dioxo Cr(VI) species is predicted (Tables S7, S9, and S10), like in the case of the corresponding models of Cr(VI) species on pure silica.21 For more dehydrated systems, constituting the series l, this tendency is weaker (Tables S9 and S10). This can be explained mainly by the transformation of the square pyramidal monooxo Cr(VI) species (1k) to the much more stable tetrahedral monooxo species (1l) during the dehydration, although all dehydrated structures (series l) are favored at higher temperatures (Table S11). After modification of the surface with the second Al site (series m, corresponding to the series k), the

Table 5. Calculateda and Experimental Vibrational Frequencies (cm−1) for the Monooxo and Dioxo Cr(VI) Species on AlModified Silica Al/Crb

monooxo model

1 1 2 2 2 2 1 2 2 2

I

1B 1bI 1C 1c 1DI 1dI 1eI 1fI 1g 1p exptlc

ν(CrO)

dioxo model

νas(OCrO)

νs(OCrO)

1006 1002 1002 985 1010 1002 1029 1011 1013, 1018 1006 1010−1011

I

1006 970 998 986 1009 984 1002 994−1012 1009 1003 −

952 941 938 888−906 971 971 863−993 916 988 979 978−982

3B 3bI 3CI 3cI 3DI 3dI 3eI 3fI 3g 3p exptlc

a

The frequencies are scaled by 0.9241 and 0.9149 for the periodic (PW91) and cluster (PW91/def2-SVP) calculations, respectively. bNumber of Al sites per Cr site in the model. cRefs 22 and 23. 17600


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of the monooxo to dioxo surface Cr(VI) species for the CrO3/ AlOx/SiO2 system, compared to the CrO3/SiO2 catalyst.22,23 Calculated CrO stretching frequencies for the monooxo and dioxo Cr(VI) species on the Al-modified silica surface are close to the corresponding theoretical results for the Cr(VI) species on silica,21 in agreement with the Raman spectroscopy data reported for CrO3/AlOx/SiO222,23 and CrO3/SiO218,19 systems.

Considering the monooxo Cr(VI) species, in most cases the theoretical CrO stretching frequencies are close to the experimental numbers assigned to this mode,22,23 within a few reciprocal centimeters. Larger discrepancies are observed only for two models, 1c and 1eI, which, however, represent thermodynamically unstable species (Tables 1, 3, and 4). On the other hand, for the most probable monooxo species (1DI, 1fI, 1p) the consistency between theory and experiment is excellent. Among the dioxo species included in Table 5, the models 3DI, 3dI, 3g, and 3p represent the structures with the oxo ligands noninteracting with the surface hydroxyl groups. Here, a very good agreement is observed between the calculated frequencies and the observed bands assigned to the symmetric OCrO stretching mode. Otherwise, the theoretically evaluated frequencies are significantly diminished, because of these interactions. The most outstanding species 3fI is predicted to be unstable under dehydrated conditions (Tables 1 and 4). The computed frequencies for the asymmetric OCrO stretch are lower, on average, than the theoretical CrO frequencies predicted for the monooxo species, but most of them are close to 1000 cm−1. Although the asymmetric OCrO mode was not observed for the CrO3/ AlOx/SiO2 system by Lee and Wachs,22,23 our calculations indicate that it may overlap with the monooxo CrO stretching mode. Similar results were obtained for Cr(VI) species on silica.21 The calculated frequencies for the symmetric mode involving Cr−O−Al and Cr−O−Si stretching vibrations are in the range of 832−942 cm−1 (Table S12), which is in a general agreement with the reported broad Raman band at about 860−900 cm−1, assigned to the bridging Cr−O−Al vibrations.22,23 In the case of the dioxo species with the Cr atom bonded to the Si−O−Al bridge oxygen (3DI, 3dI), the frequencies are red-shifted (804 and 741 cm−1, respectively).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b05675. Relative energies and Gibbs energies for the surface Cr(VI) species; energies and Gibbs energies for dehydration and interconversion reactions; optimized structures for surface Cr(VI) species; ref 60 with the complete list of authors (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +48 12 6282196. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the National Science Centre, Poland, Project No. N N204 131039 and by PL-Grid Infrastructure. Other computing resources from Academic Computer Centre CYFRONET AGH (Grant MNiSW/ IBM_BC_HS21/PK/003/2013) are acknowledged.

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REFERENCES

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CONCLUSIONS Comprehensive DFT studies on the monomeric Cr(VI) oxo species variously located on the Al-modified silica surface have been performed for the first time. A variety of systematically generated advanced periodic and cluster models, which demonstrate the Cr site heterogeneity of the amorphous surface, have been proposed. Additionally, the structure of the Brønsted acid sites accompanying the surface Al sites has been considered. The obtained models can be a good starting point for future investigations of the catalytic properties of this system. It is shown that the modification of the silica surface with Al can in the same cases affect both the geometry and the relative stabilities of the Cr(VI) species. Whereas the monooxo Cr(VI) species on silica are always bonded to the support with four Cr−O−Si linkages, adopting square pyramidal geometry,21 pseudotetrahedral monooxo Cr(VI) species can be formed in the neighborhood of two Al sites incorporated into the framework of amorphous silica. Under dehydrated conditions, the relative stabilities of such monooxo Cr(VI) species, 3-fold bonded to the surface, and the dioxo Cr(VI) species are more or less comparable, depending on the location on the surface and the model applied. On the contrary, clear thermodynamic preference for the dioxo Cr(VI) species, over the monooxo one, is predicted if the Cr centers are located on pure silica.21 Usually, a similar situation also takes place in the case of the Cr(VI) species neighboring only one Al site. These computational results explain the experimentally estimated higher ratio 17601


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