Nature and Catalytic Role of Extraframework Aluminum in Faujasite

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Nature and Catalytic Role of Extraframework Aluminum in Faujasite Zeolite: A Theoretical Perspective Chong Liu,† Guanna Li,† Emiel J. M. Hensen,*,† and Evgeny A. Pidko*,†,‡ †

Inorganic Materials Chemistry Group, Schuit Institute of Catalysis, and ‡Institute for Complex Molecular Systems, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands S Supporting Information *

ABSTRACT: A comprehensive periodic DFT study complemented by ab initio thermodynamic analysis was carried out to determine the speciation of extraframework aluminum (EFAl) in faujasite zeolite. The structure and stability of a wide range of mono- bi-, tri-, and tetranuclear EFAl complexes stabilized at different locations in faujasite were investigated. The thermodynamic cycles connecting these complexes were constructed involving such elementary steps as hydration/dehydration, proton transfer, and condensation reactions. Using ab initio thermodynamics analysis it was predicted that, during high-temperature zeolite activation, the EFAl species self-organize into cationic clusters with more than one Al center. The resulting tri- and tetranuclear clusters are preferentially stabilized inside the small sodalite cages of faujasite that provide a favorable coordination and charge-compensation environment for the large multiply charged cationic clusters. The presence of such cationic EFAl clusters inside the inaccessible sodalite cages strongly enhances the protolytic propane cracking activity of vicinal supercage Brønsted acid sites. KEYWORDS: extraframework aluminum, faujasite, acid catalysis, DFT, ab initio thermodynamic analysis, alkane cracking species.11 Mota et al. carried out DFT calculations to compare the stability of six different types of mononuclear EFAls interacting with a T6 cluster model of a zeolite and identified [Al(OH)]2+ as the preferred structure.12,13 It was proposed that the interaction of such a cation with a vicinal BAS leads to enhanced acidity by stabilizing the conjugated base site at the zeolite lattice via hydrogen bonding. The proximity of EFAl species and BAS in dealuminated zeolite Y has been investigated by a combination of solid-state NMR and DFT calculations.9,10 The promoting effect of the Lewis acidic neutral Al(OH)3 and cationic [Al(OH)]2+ species located in the faujasite supercage and sodalite cages on zeolite acidity were discussed. Octahedral [Al(OH)2(H2O)4]+ complexes containing two types of H2O ligands denoted as “rigid” and “mobile” were proposed as the EFAl in hydrated dealuminated zeolite Y.14 The presence of multinuclear EFAl clusters was also considered.15 Such species may act as the Lewis acid sites, enhancing the zeolite acidity via the polarization of vicinal BAS.16−18 An alternative hypothesis about enhanced acidity due to EFAl species has been put forward by Iglesia et al.19 They proposed that the promoting effect of EFAl species is mainly associated with the decrease of the effective size of the supercage voids, resulting in tighter confinement and, accordingly, more efficient stabilization of the transition states. Lercher and co-workers20 proposed that the enhanced activity of EFAl-containing HZSM-5 zeolites is mainly due to the

1. INTRODUCTION Zeolite Y with faujasite topology is widely employed as an acid catalyst in fluid catalytic cracking (FCC) and hydrocracking processes in oil refineries.1 The catalytic properties of zeolite Y stem from the strong acidity of the protons that compensate the negative charge of the zeolite framework due to isomorphous substitution of Si4+ with Al3+.2 Faujasite zeolites are typically crystallized at Si/Al ratios below 3. As a consequence of the high concentration of framework Al (AlF), the intrinsic acidity of the bridging hydroxyl groups (Brønsted acid sites, BAS) is low and the zeolite shows limited hydrothermal stability. To improve on these aspects, usually a steam-calcination treatment is employed, resulting in the removal of part of AlF from the lattice to form extraframework aluminum (EFAl) species.3 The increased acidity is mainly due to the increased framework Si/Al ratio. In addition, it has been widely reported that the EFAl species play a role in further enhancing the acidity of the (BAS).4−6 For instance, it was found that an increasing amount of EFAl sites relative to the BAS in zeolite Y resulted in a pronounced increase of the rate of monomolecular propane cracking.7,8 The increased acidity has been associated with the synergetic action of cationic EFAl complexes and vicinal BAS.9,10 Although there is substantial evidence for the promoting effect of EFAl species on the Brønsted acidity and the resulting catalytic performance of faujasite, the structure of these complexes and the way they enhance acidity of BAS remain moot. Various mononuclear cations such as Al3+, [Al(OH)]2+, [Al(OH)2]+, and [AlO]+ and neutral clusters such as AlOOH, Al(OH)3, and Al2O3 have been proposed as intrazeolite EFAl © 2015 American Chemical Society

Received: July 15, 2015 Published: October 20, 2015 7024

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influence of the preferred EFAl species on the catalytic performance of BAS toward propane cracking reaction was further evaluated.

changes in the local environment in the vicinity of BAS rather than polarization effects that would affect the intrinsic acidity. These hypotheses are counter to the results of a spectroscopy study by van Bokhoven and co-workers21 on the dealumination of NH4Y zeolite during high-temperature steaming. Steamactivation treatment was shown to result in EFAl species at the SI′ sites located in small inaccessible sodalite cages of faujasite (Figure 1a).

2. METHODOLOGY 2.1. DFT Calculations. The DFT calculations were performed using the Vienna ab initio simulation package (VASP).32−35 The generalized gradient approximated PBE exchange-correlation functional was used.36 To account for the van der Waals interactions, the dispersion-corrected DFT-D2 method was employed.37 The electron−ion interactions were described by the projected augmented waves (PAW) method.38,39 The Brillouin zone sampling was restricted to the Γ point.40 The energy cutoff was set to 500 eV. The convergence was assumed to be reached when the forces on each atom were below 0.05 eV/Å. A modest Gaussian smearing was applied to band occupations around the Fermi level, and the total energies were extrapolated to σ → 0. Zeolite faujasite was modeled by a rhombohedral unit cell (Figure 1b).41 Although in practice the distribution of Al in the zeolite framework will depend on the exact synthesis conditions,42,43 we have constructed our models on the basis of stability considerations. The locations of the framework Al atoms were selected by minimizing the total energy of the system with respect to the AlF distribution. The Si/Al ratio of the zeolite model was chosen so as to correspond to the starting zeolite employed in our recent experimental study on model EFAl complexes in faujasite.6 The framework Si/AlF ratio of the faujasite model was 7.0, corresponding to 6 AlF per unit cell. The total Al content was varied by addition of EFAl ions, resulting in overall Si/Altotal ratios ranging between 4.2 and 6.0. Prior to the investigation of the stability of EFAl species in zeolites, we have constructed 10 EFAl-free models with different framework Al distributions and compared their stability (Figure S1, Supporting Information). The lattice charge due to the presence of framework Al was compensated by protons introduced at the O1 position (Figure S2, Supporting Information), which has earlier been identified as the preferred proton-acceptor site.44 The most stable EFAl-free model was employed as the matrix for the stabilization of different EFAl species (Figure 1b). This model contains both the isolated Al centers and the Al−O−(Si−O)2−Al sequences in the six-membered rings (SII sites) connecting the supercage (SUP) and sodalite cage (SOD). The cell parameters were optimized for the EFAl-free faujasite (Si42Al6O96H6) and were used for all other models. The optimized lattice parameters were as follows: a = b = c = 17.441 Å, and α = β = γ = 60.0°. Different EFAl complexes were introduced into this model to different extraframework sites, and full geometry optimizations were performed with these fixed cell parameters. The nudged elastic band method (NEB) with improved tangent estimate was used to determine the location and energy of the transition states.45 The nature of stationary points was confirmed by determining the vibrational frequencies using the finite difference method. Small displacements of 0.02 Å were used to determine the numerical Hessian matrix. Transition states were identified by verifying the occurrence of a single imaginary frequency along the reaction coordinate. 2.2. Ab Initio Thermodynamic Analysis. The thermodynamics of the formation of different EFAls was considered by constructing thermodynamic cycles originating from the hydrated octahedral Al(OH)3(H2O)3 complex inside the FAU micropores. The following equilibrium reaction was considered:

Figure 1. (a) Topology and locations of extraframework sites (SI, SI′, SII, and SII) in faujasite zeolite. Thin lines represent the crystallographic Fd3̅m faujasite unit cell. (b) Rhombohedral faujasite unit cell employed in this study.

The structure of extraframework metal ions in zeolites has been a subject of intense debate, because it plays a decisive role in determining the chemical and catalytic properties of zeolite materials. Quantum chemical calculations provide the possibility to directly access the stability of different structures of such metal cations. A general conclusion of this approach applied to cations such as Ga, Zn, Al, Cu, and Fe embedded in MOR and ZSM-5 zeolites is that mononuclear oxygenated complexes are usually less stable than oligonuclear oxygenated complexes.22 Such thermodynamic analysis points out that the driving force for the self-organization of oxygenated and hydroxylated mononuclear cations into binuclear or oligonuclear complexes is the more favorable coordination environment of the extraframework metal centers in larger clusters. For Fe- and Ga-containing zeolites, it has been demonstrated that binuclear cationic complexes are also more active for a variety of reactions.23−27 Previous investigations on the structure and nature of EFAl species in zeolites mainly considered mononuclear Al-containing complexes. On the basis of the above considerations, we expect that there will also be a driving force for the formation of multinuclear oxo/hydroxide EFAl complexes. It relates to the high basicity of terminal oxygencontaining groups and the coordinative unsaturation of Al centers in such structures.22 In this contribution, we report results of a comprehensive periodic DFT study on the structure, stability, and location of various Al-containing complexes in faujasite zeolite. The distribution of mononuclear EFAl species and their selforganization into multinuclear Al oxo/hydroxide complexes were investigated. We link the DFT data to realistic conditions by the approach outlined by Reuter and Scheffler.28,29 We have previously successfully employed this ab initio thermodynamic analysis approach to cationic complexes in zeolites.30,31 The 7025

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Figure 2. Optimized structures of mononuclear EFAl species stabilized at their most favorable locations in faujasite.

HH2O(0 K, p0), SH2O(T, p0) and SH2O(0 K, p0) in thermodynamic tables were employed.46 Then, the Gibbs free energy can be rewritten as

[Al(OH)3 (H 2O)3 , FAU] ⇌

(6x − m) 1 x−1 [Al x Om Hn, FAU] + FAU + H 2O x x x (1)

ΔG = ΔE +

The Gibbs free energy (ΔG) of formation of a particular EFAl species is then defined as ΔG =

(5)

where ΔE is the reaction energy for eq 1 calculated on the basis of electronic DFT total energies. 2.3. 27Al NMR Calculations. The NMR shielding tensors were calculated using a procedure previously successfully employed by Dědeček et al.47 The calculations were carried out using the cluster models constructed from the optimized periodic structures. The gauge-independent atomic orbital (GIAO) method48 as implemented in Gaussian 09 D.01 was employed.49 The calculations were performed using the B3LYP hybrid exchange-correlation functional50−52 and the pcS-n basis sets of Jensen53 with pcS-4 for the Al atoms and pcS-1 for all other atoms. The 27Al isotropic chemical shifts were obtained by a shift factor of 563 ppm, which converts the GIAO calculated NMR shielding 502 ppm to the experimentally measured chemical shift of 61 ppm for the EFAl-free faujasite.6 The calculated 27Al NMR shifts of different EFAls are given in Table S3 in the Supporting Information.

1 x−1 6x − m G[AlxOmHn,FAU] + G FAU + G H 2O x x x − G[Al(OH)3(H2O)3,FAU]

(2)

In the above equation, G[AlxOmHn,FAU] and G[Al(OH)3(H2O)3,FAU] are the Gibbs free energies of EFAl-modified faujasite containing AlxOmHn and Al(OH)3(H2O)3, while GFAU is the Gibbs free energy of EFAl-free faujasite. Here we assume that the entropy change of the solids is negligible in comparison with that of the other reactants, and the calculated electronic DFT energy (E) is used to represent the Gibbs free energy of the solids. For gaseous water, GH2O equals the chemical potential of water (μH2O), which is computed as μH O(T , p) = E H2O + ΔμH O(T , p) 2

(3)

2

where the change of water chemical potential (ΔμH2O) is defined as

3. RESULTS AND DISCUSSION 3.1. Mononuclear EFAl. Candidate mononuclear cationic EFAl species in faujasite include [Al(OH)2]+, [AlO]+, [Al(OH)]2+, and Al3+. In addition, we also considered neutral complexes such as Al(OH)3(H2O)3, Al(OH)3, and AlOOH.11 DFT calculations were carried out to evaluate the preferred structures and locations of these EFAls in faujasite (Table S1, Supporting Information). Figure 2 shows the structures of EFAls formed at their most favorable locations. The neutral EFAl complexes do not exhibit any significant difference in relative stability on coordination to different FAU sites. The efficient stabilization of the cationic EFAls necessitates their direct interaction with lattice charge-compensating [AlO2]− units. The stability of monovalent [Al(OH)2]+ does not depend on the Al distribution in the framework. On the other hand, the [AlO]+ ion with the same formal charge preferentially occupies the SII site that allows the formation of a

ΔμH O(T , p) = ΔμH O(T , p0 ) + RT ln(pH O /p0 ) 2

2

2

0

= HH2O(T , p ) − HH2O(0 K, p0 ) − T[SH2O(T , p0 ) − SH2O(0 K, p0 )] + RT ln(pH O /p0 ) 2

6x − m [ΔμH O(T , p0 ) + RT ln(pH O /p0 )] 2 2 x

(4)

In this way, ΔμH2O includes free energy contributions that depend on the temperature (T) and partial pressure of water (pH2O). The T and pH2O dependency of the chemical potential is obtained from the differences in the enthalpy (H) and entropy (S) of H2O molecules with respect to the reference state at 0 K. For standard pressure (1 atm), the values of HH2O(T, p0), 7026

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ACS Catalysis strong coordination bond between the highly basic extraframework oxo ligand and lattice AlF site. The cations bearing higher formal charges, such as [Al(OH)]2+ and Al3+, also preferentially occupy the SII site, providing both the favorable coordination environment and a direct charge compensation with lattice anions. With the exception of the octahedral Al(OH)3(H2O)3, all mononuclear EFAls exhibit a tetrahedral configuration of the extraframework Al (AlEF) center provided by both the extraframework (OEF) and framework oxygens (OF). Secondary interactions between the OEF and lattice AlF centers additionally stabilize the EFAl species. Such interactions are observed for all neutral and monovalent mononuclear EFAls. Selected geometrical parameters are summarized in Table S2 in the Supporting Information. The optimized AlEF−OF bond lengths in these mononuclear EFAls range from 1.815 to 2.010 Å. The OEF−AlF bonds in AlOOH and [AlO]+ are relatively shorter (1.742 and 1.793 Å) than AlEF−OF bonds, indicating a strong basicity of terminal oxygen in these EFAls. 3.2. Multinuclear EFAl. We further considered the possibility of the formation of larger aggregated EFAl species in FAU micropores. We propose that larger cationic structures can potentially be formed during the thermochemical zeolite activation via migration and self-organization of mononuclear EFAls. Here the partially dehydrated [Al(OH)2]+ cation was assumed as the starting point for the formation of such clustered species as a representative and stable mononuclear EFAl complex.22 [Al(OH)2]+ can be stabilized at different locations inside the zeolite pores. The stabilization effect is mainly provided by the coordination interaction of the AlEF center with the lattice OF atoms. We have assessed the possibility of the migration of [Al(OH)2]+ from its preferred position in the vicinity of the lattice [AlO2]− anion along the zeolite pore (Figure 3). The migration of [Al(OH)2]+ to a neighboring [SiO2] unit proceeds with a moderate barrier of 69 kJ/mol, and the subsequent transition between the two [SiO2] sites has a barrier of only 46 kJ/mol. These data point to a substantial mobility of [Al(OH)2]+ inside the faujasite pore. Such mobility of mononuclear cations is a prerequisite for their self-organization into multinuclear clusters. Below we consider condensation of [Al(OH)2]+ into bi- and trinuclear complexes and compare their stability at different locations in faujasite. In addition, the structural properties and thermodynamics of the formation of larger tetranuclear [Al4O6] clusters at different extraframework sites are considered. Figure 4 summarizes the results of DFT calculations on the dimeric and trimeric species. Independent of the location inside the zeolite pore, the self-organization of two isolated [Al(OH)2]+ into binuclear [HOAl(μ-OH)2AlOH]2+ is a highly exothermic process. The calculated reaction energies for the formation of the binuclear complex at the SII (supercage), SIII (supercage), and SI′ sodalite cage sites are −107, −122, and −110 kJ/mol, respectively. In all configurations, the AlEF centers show a distorted-tetrahedral or trigonal-bipyramidal coordination. The structures have similar AlEF−OF distances ranging between 1.855 and 2.190 Å (Table S2 in the Supporting Information). The reaction energy for the assembly of three isolated [Al(OH)2]+ into [Al3O6H6]3+ depend strongly on the location of the trinuclear complex in the zeolite (Figure 4). The reaction is only strongly exothermic, when the trinuclear cluster is formed at the cation sites of the sodalite cage (ΔE = −122 kJ/

Figure 3. Reaction energies (ΔE, kJ/mol) and activation barriers (ΔE⧧, kJ/mol) for the stepwise migration of [Al(OH)2]+ along the supercage wall.

mol). The formation of the trinuclear complex at the supercage SII and SIII sites is much less favorable (ΔE = 33 and −25 kJ/ mol, respectively). We attribute this to a more favorable coordination of all Al3+ cations created by the confined environment of the sodalite cage in comparison with the location in the larger supercage. In all configurations of [Al3O6H6]3+, one of the AlEF atoms only coordinates with the extraframework hydroxyl groups, whereas the other two AlEF atoms interact with zeolite OF atoms (AlEF−OF) with distances ranging between 1.845 and 2.043 Å (Table S2 in the Supporting Information). Finally, we considered the structures and stabilities of a larger four-nuclear EFAl. We assumed a symmetric [Al4O6] species as the starting model representing a neutral oxide-like species. As the number of candidate reaction paths that connect mononuclear Al complexes to the [Al4O6] cluster is very large, we only considered the stability of this tetranuclear complex at different cation-exchange sites. Selected geometry parameters of different optimized tetranuclear clusters in faujasite are given in Table S2 in the Supporting Information. [Al4O6] has a tetrahedral structure with Al atoms at the vertexes connected by bridging oxygen atoms. The optimized structures of faujasite models stabilizing such a cluster at different extraframework sites of the zeolite are shown in Figure 5. Similar to the case of the trinuclear clusters, a much more efficient stabilization of [Al4O6] can be achieved in the smaller sodalite cage. The respective [Al4O6]SOD structure is more stable by 151 and 320 kJ/mol than its counterparts featuring 7027

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Figure 4. Optimized structures and relative stabilities of bi- and trinuclear EFAls formed by the self-organization of mononuclear [Al(OH)2]+ cations in faujasite. The reaction energies in kJ/mol correspond to the differences in total energies of the faujasite models containing multinuclear EFAl products and the respective starting configurations containing two or three isolated [Al(OH)2]+ located in a single faujasite unit cell.

The results in Figures 4 and 5 evidence a strong increase in the energy difference between the structures located at sodalite and supercage cation sites for the larger EFAl species. This suggests an increasing thermodynamic preference for the stabilization of more aggregated EFAl inside the smaller sodalite cages. 3.3. Interconversion of Different EFAl Species. The different potential EFAl complexes can interconvert via such elementary steps as hydration/dehydration, proton transfer, and condensation/self-organization. Starting from the initial mononuclear octahedral Al(OH)3(H2O)3 species the various complexes discussed in the previous section can be formed. Scheme 1 summarizes the computed energetics of the transformations of mononuclear EFAls. The dehydration of the initial octahedral Al(OH)3(H2O)3 to a tetrahedral Al(OH)3 species is an endothermic process (ΔE = 221 kJ/mol). The formation of a hypothetical AlOOH by the dehydration of Al(OH)3 further increases the total energy of the system by 208 kJ/mol. This transformation involves an intramolecular proton transfer and subsequent desorption of water (Figure S3a in the Supporting Information). The direct dehydration of Al(OH)3 to a H2O···AlOOH adsorption complex has a barrier of 108 kJ/ mol, while the subsequent water desorption step resulting in an

Figure 5. Optimized structure of [Al4O6] stabilized at different locations of faujasite. Reaction energies are given in kJ/mol.

coordination of EFAl at the SII and SIII supercage sites, respectively. Scheme 1. Transformation of Mononuclear EFAl in Faujasitea

a

Reaction energies are given in kJ/mol. 7028

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−90 kJ/mol). The [HOAl(μ-O)(μ-OH)Al]2+, [Al(μ-O)2Al]2+, and [OAl(μ-O)Al]2+ cations can be additionally stabilized by −53 to −60 kJ/mol via protonation, resulting in [HOAl(μOH)2Al]3+, [Al(μ-OH)(μ-O)Al]3+, and [HOAl(μ-O)Al]3+ complexes, respectively. Further protonation of these species is unfavorable because of the accumulation of positive charges on the EFAls that is difficult to compensate directly by the framework anionic sites. Accordingly, the formation of [Al(μOH)2Al]4+ and [HOAl(μ-OH)Al]4+ destabilize the system by 64 and 88 kJ/mol, respectively. The trinuclear [Al3O6H6]3+ complexes can also undergo transformations resulting in various EFAls (Figure S4 in the Supporting Information). Although dehydration of such clusters is endothermic, the elementary steps of the sequential water removal from [Al3O6H6]3+ to [Al3O3]3+ via [Al3O5H4]3+ and [Al3O4H2]3+ intermediates are more favorable than the respective processes for the binuclear sites. The calculated energy changes for the removal of the first, second, and third water molecules from [Al3O6H6]3+ were 50, 168, and 185 kJ/ mol, respectively. The favorability of the proton transfer from the zeolite framework to the trinuclear EFAl species depends on the structural and electronic properties of proton acceptors. Proton transfers from BAS to [Al3O4H2]3+ and [Al3O3]3+ to form the formally 4+ charged [Al3O4H3]4+ and [Al3O3H]4+ stabilize the system by 77 and 28 kJ/mol, respectively. However, further protonation of the EFAl cation resulting in [Al3O4H4]5+ and [Al3O3H2]5+ becomes endothermic by 95 and 58 kJ/mol because of the difficulty in directly compensating for the increasing positive charge of the extraframework by distantly distributed framework [AlO2]−. Protonation of the neutral [Al4O6] cluster by proximate zeolite BAS can lead to the formation of alternative charged tetranuclear EFAls (Figure S5 in the Supporting Information). These processes are strongly favored thermodynamically. The [Al4O6] can exothermically accept up to 5 H+ from vicinal BAS via sequential protonation of bridging OEF atoms, resulting in cationic [Al4O6Hn]n+ (n = 1−5). The sixth H+ transfer would be slightly unfavorable. The calculated energies for the respective elementary protonation steps are −99, −26, −50, −39, and −33 kJ/mol. Further protonation of [Al4O6H5]5+ resulting in [Al4O6H6]6+ is endothermic by 6 kJ/mol. Summarizing, DFT calculations reveal that for all of the models considered in this work the transformations of EFAls via dehydration are endothermic. The energy losses associated with the removal of water can be partially compensated by protonation with neighboring BAS and entropy gain due to the formation of water in the gas phase. The extent of protonation depends on the charge of the EFAl complex. In most cases, the EFAl structures show high affinity for protonation due to the high basicity of terminal oxygen-containing groups. Upon increasing degree of protonation, the increasing charge will result in destabilization of the EFAl species. 3.4. Ab Initio Thermodynamic Analysis. Electronic structure calculations allow assessing the energetics of the potential EFAl species under vacuum at 0 K, which show that the mononuclear Al(OH)3(H2O)3 has the lowest energy (Table S2 in the Supporting Information). However, under the conditions of steam activation of faujasite catalysts, the effect of temperature and the presence of steam may introduce a substantial entropic contribution that affects the energetics and stability of the EFAl species. To analyze the stability of EFAls under realistic reaction conditions, a statistical thermodynamic analysis was made following established

AlOOH structure is endothermic by 143 kJ/mol. The energy loss associated with the elimination of water can be partially compensated by the formation of cationic species via protonation of the complex by zeolite BAS. The protonation of AlOOH and the formation of [Al(OH)2]+ species is exothermic by −145 kJ/mol. The direct dehydration of this state leads to much less stable [AlO]+ and Al3+ (ΔE = 256 and 259 kJ/mol, respectively). An alternative, more favorable conversion path involves the protonation of [Al(OH)2]+ by a zeolite BAS followed by water removal to form H2O··· [Al(OH)]2+ (Figure S3b). The activation barrier for this reaction is only 77 kJ/mol, which is substantially lower than the enthalpy of the next water desorption step yielding [Al(OH)]2+ (ΔE = 97 kJ/mol). These data suggest that the interconversion of EFAls inside the zeolite pores is largely controlled by thermodynamics rather than kinetics. Figure 6 shows the transformation of [HOAl(μOH)2AlOH]2+ toward other binuclear EFAls inside the sodalite

Figure 6. Transformation of binuclear EFAl in the sodalite cage of faujasite. Reaction energies are given in kJ/mol.

cage. The dehydration of [HOAl(μ-OH)2AlOH]2+ leads to [HOAl(μ-O)(μ-OH)Al]2+, which can then be converted to [Al(μ-O)2Al]2+. These dehydration steps are endothermic with reaction energies of 171 and 239 kJ/mol, respectively. The mechanism of these overall dehydration reactions is similar to that described above for the dehydration of Al(OH)3. It involves sequential intramolecular proton transfer and water desorption (Figure S3c in the Supporting Information). The activation barrier for the intramolecular proton transfer within [HOAl(μ-OH)2AlOH]2+SOD is 94 kJ/mol. The removal of water from the resulting adsorption complex to form [HOAl(μO)(μ-OH)Al]2+ increases the total energy by 100 kJ/mol. Further dehydration toward [Al(μ-O)2Al]2+ proceeds with a comparable energy penalty. The computed barriers for the proton transfer and H2O desorption are both equal to 132 kJ/ mol. Similar to the structurally analogous [Ga2O2]2+ complexes previously considered as the active component in alkane dehydroaromatization zeolite-based catalysts,24 [Al(μ-O)2Al]2+ can isomerize into a linear [OAl(μ-O)Al]2+ complex (ΔE = 7029

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ACS Catalysis methods.28−30 The hydrated mononuclear Al(OH)3(H2O)3 was the reference state in this analysis.14 For the EFAlcontaining models with the same composition, the most stable configurations were selected for the thermodynamic analysis, and the Gibbs free energy here was considered for the transformation of 1 mol of AlEF atoms. Figure 7 reports the calculated relative Gibbs free energies (ΔG) of various EFAl species in faujasite zeolite as a function of

comparison with the initial mononuclear hydrated octahedral complex. Nonetheless, the reaction free energy differences are rather small, suggesting that a variety of multinuclear EFAls may coexist in a realistic catalyst. Experimentally, the synthesis of ultrastable Y zeolite catalysts (USY) is carried out at T ≥ 773 K and pH2O ≤ 1 atm.54 Under such conditions, the mononuclear [Al(OH)]2+ and trinuclear [Al3O4H3]4+ are the most stable species. Note that this analysis assumes that [Al(OH)]2+ is located at its most preferred cationic site, while the DFT calculations evidenced that its stability is very sensitive to the distribution of framework Al in terms of the charge compensation effect. When the concentration of such mononuclear species increases above 1 per unit cell, the system becomes effectively destabilized due to the placement of the EFAl complex at less favorable cationic position sites. For example, the stabilized [Al(OH)]2+ at the SIII site can raise the system energy by 168 kJ/mol in comparison to the most stable configuration containing this EFAl complex at the SII site. In such case, the self-organization into a more stable multinuclear complex is very probable. The trinuclear [Al3O4H3]4+, bearing a high formal charge, still shows high stability, even though some framework Al atoms for charge compensation are located distantly. The experimental study on dealumination of zeolite Y by van Bokhoven and co-workers21 showed that the migration of framework Al3+ to extraframework sites mainly occurs at 450− 500 K. The formed EFAl species were observed to occupy sodalite cages. Our thermodynamic analysis at a temperature of 500 K is in line with those experimental observations. The multinuclear EFAls show high stability over a wide range of partial water pressure. The aggregated EFAls such as tri- and tetranuclear clusters are preferentially stabilized inside the sodalite cage, which provides an optimal confinement environment due to its smaller void in comparison to a supercage. The exact speciation of EFAl in steam-calcined faujasites will also depend on factors such as the synthesis and treatment history of the sample. The present results point to the importance of the thermodynamic conditions in stabilizing particular EFAl complexes inside the zeolite pores. When we assume that there are no kinetic limitations during prolonged steam-calcination treatment procedures, we predict multinuclear cationic complexes to dominate among the EFAl species. 3.5. 27Al NMR Calculations. Experimentally, the state of aluminum in zeolites is usually characterized by solid-state 27Al MAS NMR spectroscopy. The 27Al NMR spectra of dehydrated and hydrated faujasite zeolites show several peaks assigned to different aluminum species.7,14,16,55,56 Generally, the three peaks at around 60, 30, and 0 ppm are assigned to fourcoordinated framework Al, five-coordinated EFAl, and sixcoordinated EFAl, respectively. To further substantiate the theoretical findings above and facilitate their comparison with the experimental findings, we simulated the 27Al NMR spectra for selected representative structures (Table S3 in the Supporting Information). Independent of the specific location in the lattice, all framework Al atoms in EFAl-free zeolite models were characterized by the same calculated shielding values corresponding to the experimentally measured chemical shift of 61 ppm.6 The calculated chemical shifts for tetrahedral AlF centers in EFAl-containing models were in the range 58−62 ppm. The extraframework Al atoms with trigonal-bipyramidal

Figure 7. Calculated relative Gibbs free energies (ΔG) of EFAl species in faujasite zeolite as a function of water chemical potential (ΔμH2O): (a) values of ΔμH2O are scaled into water pressure at different temperatures; (b) magnified area indicated in (a) with a dashed line. For illustrative purposes the ΔμH2O scale in (b) is converted to temperatures with a water partial pressure of 1 atm.

water chemical potential (ΔμH2O). At higher values of ΔμH2O (>−0.62 eV), Al(OH)3(H2O)3 exhibits the lowest Gibbs free energy among the structures considered, suggesting the predominance of this octahedral mononuclear complex in hydrated faujasite prior to calcination. However, with decreasing ΔμH2O, the relative stability of multinuclear EFAls increases. Regarding the experimental conditions (for example, at pH2O = 1 atm and T = 600−900 K, ΔμH2O = −1.80 to −1.11 eV), bi- and trinuclear species are more stable by 1−4 eV in 7030

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ACS Catalysis

Figure 8. (a) EFAl-free (H-FAU) and EFAl-containing (EFAl/H-FAU) faujasite models used for propane cracking reactions. (b) Reaction mechanism of the rate-determining step of monomolecular propane cracking by protonation of the C−C bond. (c) Local structures of adsorption complexes (Ads), transition states (TS), and reaction intermediates (Int) for propane cracking in H-FAU and EFAl/H-FAU models. (d) Reaction energy diagram of propane cracking in faujasite zeolites.

and square-pyramidal configurations gave higher field shielding with chemical shifts in the range 25−31 ppm, while the neutral octahedral Al(OH)3(H2O)3 in the sodalite cage was characterized by a chemical shift of 19 ppm. The presence of charged [Al(H2O)6]3+ in faujasite was also considered, and it was found that this fully hydrated species can only be stabilized inside the sodalite cage. The calculated 27Al NMR shift for [Al(H2O)6]3+ is 3 ppm, which is in very good agreement with the experimental value of 0 ppm for octahedral hydrated Al species. The placement of [Al(H2O)6]3+ in the supercage leads to spontaneous proton transfer to zeolite framework oxygen atoms, resulting in the formation of partially charged species such as [Al(OH)(H2O)5]2+ and [Al(OH)2(H2O)4]+ with calculated 27Al NMR shifts of 11 and 20 ppm, respectively. In experiments, the hydration of calcined zeolite Y leads to the formation of octahedrally coordinated Al species in the 27Al MAS NMR spectrum at 0 ppm.50,51 To account for hydration of EFAl clusters inside zeolite micropores, several water molecules were adsorbed onto [Al 3O4H3]4+ containing faujasite, and the 27Al NMR shift calculations were performed on the optimized structures (Figure S6 and Table S3 in the Supporting Information). The water molecules can be coordinated to framework or extraframework Al to form octahedral Al, which gave very similar chemical shifts (5 and 9 ppm). These shifts are very close to the experimental values for octahedral Al (0 ppm). Accordingly, the experimentally observed octahedral EFAls in hydrated faujasite can also be multinuclear complexes. 3.6. Catalytic Role of EFAl. The ab initio thermodynamic analysis indicates that the multinuclear cationic complexes are among the most stable EFAl species in faujasite under experimentally realistic conditions. Such tri- and tetranuclear clusters are preferentially formed in smaller inaccessible sodalite cages rather than in the large supercages. Because of such locations, the promoting role of EFAl on the catalytic

performance of faujasite zeolites cannot be explained by the direct interaction between the reactants and EFAls. To get an insight into the effect of such species on the catalytic properties of Brønsted acid sites in faujasite, we evaluated and compared the energetics of monomolecular propane cracking reactions over the EFAl-free (H-FAU) and EFAl-containing (EFAl/HFAU) zeolite models (Figure 8a). The EFAl/H-FAU model contains a trinuclear [Al3O4H3]4+ cluster inside the faujasite sodalite cage with a vicinal supercage Brønsted acid site. The monomolecular propane cracking reaction involves the protonation of the C−C bond in C3H8 by a BAS, resulting in a separate ion pair containing a carbonium ion intermediate and a deprotonated anionic zeolite site (Figure 8b).57,58 At the next step, the carbonium ion decomposes to yield the methane and ethylene cracking products. The initial protonation step and the formation of the carbonium ion determines the overall rate of the cracking process.57 Figure 8c shows the local structures of the adsorption complexes (Ads), transition states (TS), and carbonium ion intermediates (Int) for the C−C bond activation on H-FAU and EFAl/H-FAU models. The structures of Ads are similar for both faujasite models, and the computed adsorption energies of propane in H-FAU and EFAl/H-FAU are −49 and −57 kJ/ mol, respectively. The presence of EFAl influences strongly the structures of TS and Int. The length of the protonated C−C bond in the TS formed in EFAl/H-FAU model is shorter by 0.14 Å than that in the case of the H-FAU model, while that in the carbonium ion Int in the EFAl/H-FAU model was 0.15 Å longer than that in the EFAl-free model. The respective reaction energy diagram of propane cracking is presented in Figure 8d. The intrinsic barriers for propane cracking are 200 and 149 kJ/mol for the H-FAU and EFAl/HFAU models, respectively. The final state of the C−C cracking Int is also significantly stabilized by the presence of the EFAl species (Figure 8d). The analysis of the geometries of TS and 7031

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ACS Catalysis Int reveals a stronger separation of the [C3H9]+ carbocation and the anionic structures in EFAl/H-FAU in comparison to the situation in the EFAl-free H-FAU model. Assuming that the effect of the zeolite structure on the stability of carbocation is minor, the strong stabilization of the charge-separated TS and Int states in the former case is due to the additional compensation of the negative charge of the deprotonated lattice sites by the multiply charged cationic EFAl cluster. The decrease of the computed activation barrier for propane cracking points to the ability of multinuclear EFAl cations in sodalite cages to enhance the catalytic activity of a proximate Brønsted acid in faujasite zeolite. This is in line with earlier suggestions on the promoting role of cationic EFAl species on the catalytic activity of acidic zeolites.59−61

CAM, FP7-NMP-2013-EU-Japan-604319) is acknowledged. NWO is acknowledged for access to supercomputer facilities.

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4. CONCLUSION The structural properties, stability, and role of EFAl species for alkane cracking over faujasite-type zeolites were studied by periodic DFT calculations combined with ab initio thermodynamic analysis. A wide range of mononuclear as well as bi-, tri-, and tentranucelar EFAl complexes stabilized at different locations in faujasite micropores was investigated. DFT calculations point to a strong thermodynamic preference of the mononuclear species to self-organize into cationic clusters of higher nuclearity. The resulting tri- and tetranuclear clusters are preferentially stabilized inside the small sodalite cages of faujasite that provide a favorable coordination and chargecompensation environment for the large multiply charged cationic clusters. The interconversion pathways of various EFAl species were constructed involving such elementary steps as hydration/dehydration, proton transfer, and condensation reactions. The effect of temperature and gaseous water pressure was accounted for by using ab initio thermodynamics, which indicates that the stability of multinuclear EFAl species significantly increases under the realistic catalyst preactivation conditions. We expect that the EFAl speciation during prolonged steam-calcination procedures will be thermodynamically equilibrated; in such case, multinuclear cationic complexes are predicted to be the dominant EFAl complexes in activated faujasite zeolite. The presence of multiply charged cationic EFAl clusters inside the inaccessible sodalite cages enhances the catalytic reactivity of vicinal supercage Brønsted acid sites, as evidenced by a strong decrease of the intrinsic barrier of monomolecular propane cracking.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b02268. Selected geometrical parameters, energetics, and calculated NMR data for optimized zeolite structures (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail for E.J.M.H.: [email protected]. *E-mail for E.A.P: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.L. thanks the China Scholarship Council (CSC) for financial support. Financial support by the European Union (NOVA7032

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