Desilication of SAPO-34: Reaction Mechanisms from Periodic DFT

Article pubs.acs.org/JPCC

Desilication of SAPO-34: Reaction Mechanisms from Periodic DFT Calculations Torstein Fjermestad,† Stian Svelle,† and Ole Swang*,†,‡ †

inGAP Center for Research-Based Innovation, Department of Chemistry, University of Oslo, P.O. Box 1033, Blindern, 0315 Oslo, Norway ‡ SINTEF Materials and Chemistry, P.O. Box 124, Blindern, 0314 Oslo, Norway S Supporting Information *

ABSTRACT: With the aim of understanding the desilication of SAPO-34, we compared three different reaction mechanisms for the hydrolysis of framework silicon by use of density functional theory (DFT) calculations. All three mechanisms are characterized by stepwise hydrolyses of Si−O−Al bonds. In the most favorable mechanism water molecules adsorb strongly to the Lewis acidic Al atoms neighboring the Si atom. Furthermore, evaluation of free energies reveals that an additional water molecule may catalyze the hydrolysis of the first Si−O−Al bond.

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formation of a siliceous phase. Barger et al.18 studied the effect of steaming SAPO-34 for 50 and 100 h at both 723 and 923 K. They observed a loss of active sites and a formation of Si islands after the steam treatment. Aramburo et al.19 steamed SAPO-34 while increasing the temperature from 393 to 973 K (5 K min−1, 180 min). They observed formation of Si islands and a reduction in the number of acid sites. The mobilization of the T atoms could happen through a hydrolysis of the O−Si/Al/P bonds by available water molecules. Water molecules in the SAPO material may have several origins: (1) Water is a byproduct in the MTO reaction. (2) In the catalyst regeneration process, water is produced through the burn-off of coke in an oxygen atmosphere. (3) In zeolites, terminal OH groups are present at the external surface or in defects of the crystal. Upon thermal treatment, these OH groups are known to dehydroxylate to form water.20 Terminal OH groups are also present in SAPO materials,21,22 and we expect the same process to occur in SAPO materials as well. This source of water is relevant in the cases where the SAPO material is heated without the presence of “external” water (e.g., the study by Buchholz et al.17). Because the material must be neutral, the formation of Si islands is accompanied by a loss of active Brønsted acid sites.23,24 The remaining Brønsted acid sites, located at the border of the Si islands, have been suggested to increase their acid strength compared to the acid sites of isolated Si atoms.25,26 A recent computational work suggests, however, that the acid strength of the two different Brønsted sites is comparable.23 The loss of Brønsted active sites is detrimental to the catalytic properties of the material.18,19,27,28 An improved understanding of the mechanism(s) of silicon island formation could therefore be a help in designing more robust catalysts.

INTRODUCTION Motivated by the rising oil prices the past decade, there has been a renewed interest in the methanol-to-olefins (MTO) process.1 One advantage of using methanol as feedstock is that it can be produced from renewable biomass as well as from natural gas. The MTO process is catalyzed by zeotype materials such as H-SAPO-34 and H-ZSM-5, and in spite of its success, the commercialization of the process still poses some challenges with respect to selectivity and catalyst deactivation.1 To optimize the product selectivity and inhibit deactivation processes, detailed knowledge of the interaction between the catalytic material and the intermediates along the pathway from methanol to olefins is desirable. Much research has been focused on the mechanism of transformation of the reactant (methanol) to the products (olefins), resulting in an impressive level of mechanistic detail. The mechanism is not a direct conversion of methanol molecules to olefins.2 Rather, the MTO transformation is catalyzed by active hydrocarbon species present in the cages of the zeolite material. The active hydrocarbon species undergo methylations and olefin elimination.3−15 Contrasting the mechanistic research on the MTO transformation, there has been much less focus on understanding how the catalytic material behaves during the process. Insight into the dynamics of the material is crucial for an improved catalytic design. For SAPO materials several observations of T atom (tetrahedrally coordinated atom) mobility have been reported. Under the influence of heat and/or humidity the isolated Si atoms tend to cluster together to form Si islands in the material. Vomscheid et al.16 studied the stability of H-SAPO-34 exposed to a humid atmosphere (75% relative humidity) at room temperature. After 5 days, formation of silicon islands had occurred. Buchholz et al.17 studied the thermal stability of several SAPO materials at 1173 K. After the thermal treatment, the materials were rehydrated and the NMR spectra indicated © 2015 American Chemical Society

Received: October 29, 2014 Revised: December 18, 2014 Published: January 9, 2015 2073

DOI: 10.1021/jp510844v J. Phys. Chem. C 2015, 119, 2073−2085

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The Journal of Physical Chemistry C

points. To describe the transition state geometries more accurately (important for the phonon calculations, see below), we refined the NEB results by climbing image nudged elastic band calculations (CI-NEB).38 For the most favored mechanism, the entropic effect of the water adsorption and the effect of dispersion corrections39,40 were investigated. The Gibbs free energy was computed for critical points optimized with dispersion corrections along the reaction path. To obtain acoustic modes (corresponding to translation of the unit cell) with frequencies close to zero, it was necessary to set the convergence threshold of the phonon calculation to 1.4 × 10−13 eV (tr2_ph = 1 × 10−14 Ry). To obtain a value of exactly zero for the acoustic modes, the acoustic sum rule was imposed. For the free water molecule, the Gibbs free energy correction was calculated as follows: The vibrational contribution was obtained by a phonon calculation. Six of the vibrational modes correspond to translations and rotations of the water molecule. By imposing the acoustic sum rule, these modes were set to zero. The translational and rotational contributions were extracted from a frequency calculation on the optimized geometry of the water molecule performed using Gaussian 09.41 The translational and rotational free energy contributions are expressed analytically, hence depending only on molecular geometry. In the calculation of the pressure volume work, the ideal gas approximation, PV = RT, was used. The free energy of the water molecule was calculated by adding the free energy correction to the potential energy. Because we wanted to model a situation corresponding to the experimental conditions of Vomscheid et al.,16 we computed the free energy of water at a temperature of 298 K and a pressure of 0.02 atm. For the SAPO-34 material, only the vibrational contribution to the Gibbs free energy correction was considered. In general, the vibrational contribution to the change in Gibbs free energy can be expressed as follows:

We envision that thorough computational mechanistic investigations may bring us closer to a useful description of the hydrothermal stabilities of microporous materials in general. In the present article, we present a study on the desilication of SAPO-34, which we assume to be the first step of silicon island formation. In addition to studies based on comparison of DFT electronic energies, we have investigated the effects of dispersion and free energy corrections for the reactions under study.

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METHODS Model of the Catalyst. The substitution of phosphorus by a silicon atom in pure AlPO-34 causes the material to be negatively charged. This negative charge is compensated by the introduction of a cation in the vicinity of the Si atom. In principle, any cation can fill this role, but for materials catalyzing the MTO process, the cation is a proton. The proton forms a Brønsted acid site by binding to any of the four oxygen atoms bonded to the Si atom. The literature is ambiguous with respect to which location is the most stable.29−32 In our previous publication33 and in this work we have placed the proton at O(2) (see Figure 1 for an explanation of the labeling

ΔG = ΔEv + Δ(PV ) − T ΔSv

(1)

where Ev is the vibrational contribution to the internal energy, P the pressure, V the volume, T the temperature, and Sv the vibrational contribution to the entropy. For a solid material the Δ(PV) term is usually small, and we assume it to be zero for our purposes. The vibrational contribution to the internal energy, Ev, can be expressed as follows:

Figure 1. A three-dimensional and two-dimensional representation of the silicon heteroatom in SAPO-34. The labeling scheme of the different crystallographic oxygen atoms is taken from Ito et al.34 The aluminum atoms are labeled according to the oxygen atom to which they are connected.

⎛1 ⎞ 1 ⎟ Ev = R ∑ Θv , K ⎜ + Θ / T v , K ⎝2 e − 1⎠ K

(2)

where R is the molar gas constant and Θv,K, is the vibrational temperature. The vibrational contribution to the entropy, Sv, is expressed as follows:

system). In the discussions on relative energies, we have chosen the zero level of the energy scale to be the sum of the energies of a free water molecule and the pure SAPO-34 material without any water adsorbed. Computational Details. Periodic density functional calculations were performed using the Quantum Espresso code.35 To model the SAPO-34 material, the orthorhombic unit cell of chabazite (36 atoms) was used. Geometries and energies were calculated using the same approach as in our earlier work.33 The reaction pathways between the intermediates were described by nudged elastic band (NEB) calculations36,37 with 10 images including the start and end

Sv = R ∑ K

e

Θv , K /T Θv , K / T

−1

− ln(1 − e−Θv ,K / T ) (3)

For the intermediates, the three acoustic modes are not included in the summations in eqs 2 and 3. For the transition states, also the imaginary vibrational mode is excluded from the summations. The vibrational Gibbs free energy correction is then added to the potential energy to obtain the total Gibbs free energy of the material. 2074


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Figure 2. Potential energy profile of pathway 1 of the desilication of SAPO-34. Energies in kJ mol−1. Some atoms are colored to indicate their origin at the start of the reaction.

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RESULTS We recently published a mechanistic comparison between the dealumination in zeolite SSZ-13 and the desilication in SAPO34.33 In that comparative study, we chose to model a reaction path for the SAPO that was as similar as possible to that already suggested for the zeolite. Figure 2 shows the potential energy profile of the desilication of SAPO-34 studied in that work. Three of the transition states, TS1−2 (118 kJ mol−1), TS2−3 (119 kJ mol−1), and TS7−8 (56 kJ mol−1), are of particularly high energy compared to preceding intermediates. Pursuing the hypothesis that the desilication of SAPOs may proceed according to reaction paths qualitatively different from the corresponding dealumination of zeolites, we have investigated a number of other candidates for such a mechanism. Two of them were found to have significantly lower activation energies. To facilitate the discussion, we label the “zeolite-like” mechanism pathway 1. Pathway 2: An Alternative Desilication Pathway for SAPO-34. The energetically unfavorable transition states of pathway 1 appear in the hydrolysis of the first and the third T− O−T bond. In our search for a more favorable desilication pathway, we therefore focused on these two steps. Hydrolysis of the First Bond. An important feature of this step for pathway 1 is the configuration of the vicinal disilanol group of structure 2 (14 kJ mol−1). This involves Si and Al4 with the Brønsted acid site at O(2) (see Figure 3). We optimized an alternative vicinal disilanol structure (13, 41 kJ mol−1, Figure 3) involving Si and Al2 with the Brønsted acid site at O(1). When connecting 13 to previous (12, −43 kJ mol−1, Figure 3) and subsequent (14, −33 kJ mol−1, Figure 3)

structures along the reaction path, transition states of lower energies results. The elementary step from 12 to 13 goes over transition state TS12−13 (43 kJ mol−1) of a much lower energy than TS1−2 (118 kJ mol−1) of pathway 1. Compared to pathway 1, the transition from 13 to 14 has a lower activation energy. It is also geometrically simpler: The shallow minimum between TS2−3 and TS3−4 has now disappeared and the transition state TS13−14 (94 kJ mol−1) is more stable than TS2−3 (119 kJ mol−1) of pathway 1. 14 differs from 4 by the position of one proton. In 14 the proton is part of the aqua ligand at Al1 while in 4 the proton is bonded to O(4) (see Figure 3). Figure 4 shows the potential energy profiles for the first hydrolysis step of pathway 1 (orange) and pathway 2 (black). The explanation for the lower energy barriers of pathway 2 might be found in the formation of 2/13 and their further reaction to 4/14. In the formation of 13 one of the protons of the adsorbed water molecule is transferred over to O(1). In 12, the same proton is strongly H-bonded to O(1) (1.70 Å, see Figure 3), and the proton transfer should not require much energy. In 1, the intermediate preceding 2, the proton to be transferred to O(4) is not involved in any H-bond. This might partially explain the higher barrier toward the formation of 2 compared to the formation of 13. In the formation of 14/4 from 13/2, the lower barrier in pathway 2 might be caused by the fewer bonds being broken: In the transformation from 2 to 4 three O−Al bonds and one O−Si bond are broken. In contrast, the transformation from 13 to 14 occurs through a breakage of one O−Al bond, one O−Si bond, and one O−H bond. 2075


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Figure 3. Intermediate structures along the hydrolysis of the Al1−O(1)−Si bond for pathways 1 and 2. The atoms are colored according to their elements: red = O, green = P, white = H, yellow = Si, and magenta = Al. The labeling scheme of the atoms is the same as in Figure 1. The oxygen atoms originating from the adsorbed water molecules are labeled Onaq where n = 1, ..., 4.

Hydrolysis of the Second Bond. The barrier of the hydrolysis of the second T−O−T bond (Al4−O(4)−Si) of pathway 1 (84 kJ mol−1) is significantly lower than the barriers of the first and third hydrolysis steps. For this reason, we did not make any effort in searching for lower energy paths. For the hydrolysis of the second bond, the mechanisms of pathways 1 and 2 therefore coincide. Upon the coordination of the water molecule to 14, the system relaxes to 5 (−82 kJ mol−1). The system then goes over TS5−6 (2 kJ mol−1) to 6 (−50 kJ mol−1) before it relaxes to 7 (−91 kJ mol−1) upon the coordination of a water molecule to the Brønsted acid site. Hydrolysis of the Third Bond. The high barrier for the hydrolysis of the Si−O(3)−Al3 bond in pathway 1 (147 kJ mol−1) is explained by the change in the coordination sphere of Al3 when going from 7 to TS7−8. In 7, all O−Al3−O bond angles have values that deviate less than 4° from the tetrahedral angle of 109.5°. When reaching TS7−8, Oa−Al3−Oc and Ob− Al3−Oc (see Figure 5) both have a value of ∼130°. For a fourcoordinated Al, such a geometry is associated with a high energy. The change in the coordination sphere of Al3, from tetrahedral (in 7) to severely perturbed tetrahedral (in TS7−8), is presumably the main cause of the high energy barrier. To avoid this energetically unfavorable reaction path, we optimized another where the water molecule coordinates to Al2

Figure 4. Potential energy profiles (in kJ mol−1) of the hydrolysis of the first T−O−T bond for pathway 1 (orange curve) and pathway 2 (black curve). Schematic structures of the intermediates are shown. Some atoms are colored to indicate their origin at the start of the reaction. Note that although 4 and 14 are similar in structure and energy, some of their corresponding atoms have different origins.

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Figure 5. 3D structure of transition state TS7−8 of pathway 1. The O− Al3−O angles have the following values: Oa−Al3−Ob = 107.1°, Oa− Al3−O3aq = 91.5°, Oa−Al3−Oc = 130.3°, Ob−Al3−O3aq = 130.6°, Ob−Al3−Oc = 103.2°, Oc−Al3−O3aq = 97.6°.

Figure 6. Potential energy profile (kJ mol−1) of the hydrolysis of the third T−O−T bond for pathway 1 (orange) and pathway 2 (black).

and hydrolyses the Si−O1aq−Al2 bond. From 7, the water molecule coordinates to Al2 (16, −63 kJ mol−1) which acquires a trigonal bipyramidal coordination. The aqua ligand is in the axial position and O1aq is in the equatorial position. In the transition to 17 (−50 kJ mol−1), the proton of the Al4−OH group transfers over to O1aq and forms a Brønsted acid site (see Figure 7). To compensate for the negative charge thus generated at the Al4−O− group, one of the protons of the aqua ligand at Al1 transfers over to Al4−O− and regenerates the Al4−OH group. The Brønsted site at O1aq and the aqua ligand at Al2 make the breakage of the O1aq−Al2 bond possible without involving unsaturated atoms. Upon the O1aq−Al2 breakage, the coordination of Al2 changes from trigonal bipyramidal in 17 to tetrahedral in 18 (−61 kJ mol−1). The trigonal bipyramidal and the tetrahedral coordinations are both stable geometries for aluminum, and the hydrolysis of the Al2− O1aq−Si bond is a low barrier process. The difference between TS17−18 (−26 kJ mol−1) and 7 is 65 kJ mol−1, much lower than the corresponding barrier of pathway 1 of 147 kJ mol−1 (see Figure 6). Figure 7 shows the intermediates along the hydrolysis of the third T−O−T bond for pathways 1 and 2. Hydrolysis of the Fourth Bond. The last bond to be hydrolyzed in pathway 2 is the Al3−O(3) bond. This differs from pathway 1 where the Al2−O(2) is the last bond to be hydrolyzed. Apart from hydrolyzing different bonds, the two mechanisms are qualitatively similar. From 18 a water molecule coordinates to Al3 and forms 19 (−65 kJ mol−1, Figure 8). The Al3−O(3) bond breaks through a proton transfer from the aqua ligand at Al3 via the OH group at Al4 to O(3). The system then relaxes to 11 (−93 kJ mol−1). In 11 the Si(OH)4 moiety has broken all its bonds to the surrounding Al atoms and is held in place only by H-bonds. The Si(OH)4 species may move to a defect free region of the material; this causes the potential energy to increase by 41 kJ mol−1. The potential energy profile of pathway 2 is shown in Figure 9. Pathway 3: Water Catalyzing the First Hydrolysis Step. The change from 1 (−75 kJ mol−1) to 12 (−43 kJ mol−1) in pathway 2 describes a likely mechanism at low water vapor pressures and high temperatures. At higher concentrations of water vapor and lower temperatures it might be more favorable to coordinate a second water molecule to Al2

without breaking the strong H-bond to the Brønsted acid site. A similar water coordination to SAPO materials was suggested by Buchholz et al. based on CF MAS NMR experiments.42 Upon steaming of SAPO-34 at 298 K, they first observed the disappearance of the peak corresponding to the Brønsted acid (1H NMR). Subsequently, a peak corresponding to an octahedral Al coordination sphere (27Al NMR) was observed. Because of the experimental observation and the theoretical insight achieved by considering the change from 1 to 12, we optimized structure 22 (−99 kJ mol−1, Figure 10). This structure has one water molecule coordinated to the Brønsted site and another water molecule coordinated to Al2. The potential energy of 22 is 56 kJ mol−1 lower than 12. From 22 the hydrolysis of the Al1−O(1)−Si bond proceeds along a mechanism similar to that of pathway 2 (see potential energy profile in Figure 11). The difference is that now a water molecule is coordinated to the O(2)H group of the vicinal disilanol throughout the hydrolysis of the bond. The coordination of the water molecule to the O(2)H group has a dramatic effect on the barrier of the hydrolysis of the Al1− O(1)−Si bond. The barrier height goes down from 169 to 107 kJ mol−1, and the vicinal disilanol structure (13 in pathway 2) goes from being an intermediate to become a transition state (TS22−23, 8 kJ mol−1). In addition to forming a strong H-bond to the O(2)H group, the water molecule is involved in two other H-bonds from its protons to nearby framework oxygen atoms (see Figure 10). These H-bond interactions are presumably the cause of the lowering of the barrier for the first hydrolysis. After having passed TS22−23, the system relaxes to 23 (−68 kJ mol−1). From 23, the water molecule coordinated to the O(2)H group relocates, and the structure transforms to 5 of pathways 1 and 2. From 5, pathway 3 coincides with pathway 2. The hydrolysis of the first T−O−T bond (Al1−O(1)−Si) involves one water molecule in pathway 2 whereas in pathway 3 it involves two water molecules. Therefore, a fair comparison of the two mechanisms should be based on the Gibbs free energy. At the risk of getting ahead of ourselves (the effects of free energy corrections are described in detail below), the Gibbs free energy of TS22−23 (dispersion corrected) is 96 kJ mol−1 (see Figure 13), while the Gibbs free 2077


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Figure 7. Intermediate structures along the hydrolysis of the third T−O−T bond of pathways 1 and 2. In pathway 1 the Si−O(3)−Al3 bond is broken whereas in pathway 2 the Si−O1aq−Al2 bond is broken.

energy of TS13−14 is significantly higher (133 kJ mol−1, dispersion corrected). Hence, a picture emerges that pathway 3 is favored over pathway 2. Because pathway 3 appears to be the favored mechanism among those investigated, it will be our exclusive focus in the following. Pathway 3 with Dispersion Corrections Included. While not being described by contemporary mainstream density functionals, dispersion forces are a potentially important contribution to adsorption energies of molecules on solid surfaces.43−45 To investigate the effect of dispersion forces for the reactions presently under study, we computed the adsorption enthalpy of water on H-SSZ-13 with (ΔHads,SSZ13,disp = −100 kJ mol−1) and without (ΔHads,SSZ13= −75 kJ mol−1) dispersion corrections. The experimental adsorption enthalpy of water on H-ZSM-5, ΔHads,ZSM5,exp, is 90 ± 10 kJ mol−1.46 The different topologies (CHA and MFI) complicate the comparison between the theoretical and experimental values. However, Brändle et al. computed the heats of deprotonation of several zeolite topologies and found that H-SSZ-13 (1190 kJ mol−1) is more acidic than H-ZSM-5 (1200 kJ mol−1).47 Water should therefore adsorb more strongly to H-SSZ-13 than to H-ZSM-5. The order of the three adsorption enthalpies is as follows: ΔHads,SSZ13,disp > ΔHads,ZSM5,exp > ΔHads,SSZ13. With dispersion included, the adsorption enthalpy of water is higher on H-SSZ-

13 than on H-ZSM-5, in agreement with the work of Brändle et al.47 This clearly indicates the importance of including dispersion correction in the description of water adsorption on zeotype materials. The potential energy profile of pathway 3 with dispersion correction included is given in Figure 12. Including the dispersion correction led generally to small changes in the reaction barriers. In the forward direction, the largest change is +12 kJ mol−1. In the reverse direction, the largest change is −13 kJ mol−1. For the water adsorption energies, the change is larger and positive. These energies increase by values between 20 and 30 kJ mol−1. By comparing the energy profiles of pathway 3 with and without dispersion, it is clear that the most significant difference is in the two first water adsorption energies. With dispersion, the energy of 22 is 50 kJ mol−1 lower than what is the case when calculated without dispersion. On the other hand, when looking at the total energy change from 22 to 11, the values with (−11 kJ mol−1) and without (+6 kJ mol−1) dispersion are fairly similar. The energetics of the last step from 11 to 21 is almost identical with (+43 kJ mol−1) and without (+41 kJ mol−1) dispersion. Inclusion of Entropy Effects. When a water molecule adsorbs on the internal surface of a microporous material, the translational and rotational degrees of freedom of the water molecule are lost. Additionally, there is also a change in the 2078


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Figure 10. Structure of intermediate 22 (−99 kJ mol−1) of pathway 3. The hydrogen bonds contributing to the stabilization of the first hydrolysis are indicated.

significant contribution to the calculated free energy. On the other hand, the change in vibrational degrees of freedom upon water adsorption is much smaller. The free energy profile of pathway 3, shown in Figure 13, was found by performing phonon calculations on all stationary points along the reaction path (see Computational Details). The phonon calculation on TS19−20 only resulted in positive eigenvalues. The failure to find one negative eigenvalue for this transition state may be caused by the very flat potential energy surface in the region around TS19−20. The change from 19 to 20 is thermoneutral, and the barrier is only 18 kJ mol−1 (see Figure 12). Visual inspection of the eigenmode with the lowest real frequency (35 cm−1) indicates that it has a large component along the reaction coordinate. We are therefore confident that TS19−20 is indeed a transition state. We assumed that the lack of the negative

Figure 8. Intermediate structures along the last hydrolysis step of pathway 2.

vibrational degrees of freedom upon the adsorption of a water molecule. The loss of translational and rotational degrees of freedom represents a significant loss of entropy and therefore a

Figure 9. Potential energy profile (kJ mol−1) of pathway 2. Some atoms are colored to indicate from which water molecule they originate. 2079


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Figure 11. Potential energy profile without dispersion corrections (kJ mol−1) of pathway 3. Some atoms are colored according to the water molecules to which they initially belonged.

Figure 12. Potential energy profile (kJ mol−1) with dispersion correction for pathway 3. Some atoms are colored according to the water molecules to which they initially belonged. 2080


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Figure 13. Free energy profile of pathway 3 at 298 K and 0.02 atm vapor pressure. Energies in kJ mol−1. From 5 pathway 3 coincides with pathway 2. Some atoms are colored according to the water molecules to which they initially belonged.

we treat the modes corresponding to translations and rotations of the Si(OH)4 species as pure translations and rotations. This leads to an increase in entropy when going from 11 to 21. The computed difference between the free energy and the potential energy is therefore expected. Because of these important differences between the potential energy profile and the free energy profile, the role of the ratedetermining transition state changes in the free energy profile. From Figure 13 we see that in the free energy profile of pathway 3 TS19−20 (113 kJ mol−1) is the rate-determining transition state. The energetic span48,49 of 145 kJ mol−1 is the difference between TS19−20 and 1.

eigenvalue would not significantly affect the values of the other vibrational modes. Therefore, we approximated the free energy of TS19−20 by excluding the lowest positive eigenvalue from the summation in eqs 2 and 3. In the last structure, 21, the Si(OH)4 species is almost free to translate and rotate within the confines of the pore, taking the relevant temperature into account. We therefore modeled the modes corresponding to translations and rotations of the Si(OH)4 species as pure translations and rotations. By comparison with the potential energy profile in Figure 12, the significant difference in potential energy of adsorption and free energy of adsorption becomes evident. The values of the free energy of adsorption are from 59 to 72 kJ mol−1 less negative than the potential energy of adsorption. Contrasting the water adsorption case, the free energy profile of the hydrolysis of the Si−O−Al bonds shows a much higher similarity to the potential energy profile. When no adsorption process is involved, the only source of discrepancy between the potential energy and the free energy is the change in the vibrational degrees of freedom. Along the elementary steps of the hydrolysis pathways, this change is small. It is, however, worth mentioning some notable exceptions. The reaction free energy of the first hydrolysis, from 22 to 23, is 16 kJ mol−1, which compares with the corresponding potential energy of 38 kJ mol−1. This indicates that the vibrational contribution to the free energy stabilizes intermediate 23 relative to 22. Another notable case is the free energy change of the last hydrolysis, from 19 to 11 (−20 kJ mol−1) compared to the corresponding potential energy change of −5 kJ mol−1. The vibrational contribution to the free energy therefore stabilizes 11 relative to 19. The free energy change from 11 to 21 (−13 kJ mol−1) is significantly different from the corresponding potential energy difference (+43 kJ mol−1). In the free energy calculation of 21,

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DISCUSSION Why Is Pathway 2 More Favorable Than Pathway 1? Pathways 1 and 2 show the largest mechanistic discrepancy in the hydrolysis of the first and the third T−O−T bond. For these two hydrolyses, pathway 2 is characterized by H2O being strongly adsorbed on Al atoms neighboring the Si atom. The adsorption of H2O to Lewis acids (Al atoms in this case) increases the acidity of the protons of the water molecule.50 This increased acidity makes the subsequent proton transfers easier, and the reaction barriers of pathway 2 are therefore lower than those of pathway 1. Effects of Dispersion and Free Energy Corrections. As shown in the Results section, the dispersion and free energy corrections are most important for processes involving adsorption of water molecules on the material. For reactions of the material itself, these corrections do not lead to qualitative changes in the energy profile. Considering that the phonon calculations are relatively expensive, it might be beneficial to use cheaper methods to approximate the free energy of large systems. Salciccioli et al.51 2081


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Figure 14. Free energy profile of pathway 3 obtained with phonon calculations (black) and by removal of degrees of freedom (orange).

and Grabow et al.52 studied adsorption on metal surfaces and approximated the free energy of adsorption by removal of the translational degree of freedom (DOF). We based our approximate free energies of adsorption on this method, but in addition to the translational DOF we also removed two rotational DOF from the adsorbed water molecule. This choice is rationalized by realizing that after adsorption the water molecule can still rotate about the O---H hydrogen bond (for adsorption on an OH group) or the O−Al bond (for adsorption on an Al atom). When comparing this approximate calculation with the more accurate phonon calculation, a remarkable agreement is obtained. The free energy profiles of pathway 3 calculated with the two methods are compared in Figure 14. A significant discrepancy is, however, found for 21. In the transformation from 11 to 21 six vibrational modes are converted to three translational modes and three rotational modes. In the method of removal of DOFs, we are approximating the free energy change of a water molecule adsorbing on a SAPO material. The change from 11 to 21 is a completely different process, and the large discrepancy for 21 is therefore expected. Is Si Mobile? From the free energy profile of pathway 3 (Figure 13) the endergonic nature of the desilication of SAPO34 becomes clear. The equilibrium is highly shifted toward silicon being in the framework position. This result is in apparent contradiction to the experimental observation of the T atoms being mobile.16,18,19,53 However, the contradiction may be explained by realizing that a H3PO4 species can enter into the hydrogarnet defect generated by the desilication. As the H3PO4 species enters, the system is considerably stabilized. In the accompanying paper,54 we have calculated a detailed reaction path for the H3PO4 insertion into the hydrogarnet

defect. In that paper we show that the H3PO4 insertion is considerably more exergonic than the Si(OH)4 insertion. The Si(OH)4 species is therefore blocked from reentering into the hydrogarnet defect and remains mobile in the micropores. The dephosphoration by hydrolysis is precisely the reverse process of the H3PO4 insertion. If the dephosphoration starts from a defect or an external surface, the overall process, dephosphoration + H3PO4 insertion, leads to a stabilization of the system.54 The Si atoms are therefore mobilized to a much higher degree, in better agreement with experimental observations. The insertion of the H3PO4 into the hydrogarnet defect can be characterized as a healing of the defect. Such a healing mechanism has also been suggested based on experimental observations. From 1H, 27Al, 29Si, and 31P MAS NMR experiments, Buchholz et al.17 observed that desilication of SAPO materials did not lead to an increase in the concentration of hydrogarnet defects. Furthermore, they observed that signals due to phosphorus atoms in defect sites decreased while signal due to tetrahedrally coordinated phosphorus atoms increased. These observations clearly suggest that phosphorus species are entering into the hydrogarnet defects created by the desilication. For zeolites, an analogous mechanism has been suggested. Upon dealumination of zeolite Y, Maxwell et al. observed an increase in the Si(4Si) peak,55 indicating that a Si/Al exchange of a tetrahedrally coordinated framework atom had occurred. Campbell et al. observed that the concentration of silanol groups did not increase upon calcination or hydrothermal treatment of ZSM-5.56 Such an observation indicates that the defects generated by dealumination are healed during the heat/ steam treatment. 2082


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The Journal of Physical Chemistry C Digestif: A Short Overview of Recently Described Hydrolysis Reactions of Tetrahedral Atoms in Zeotype Materials. To facilitate further research in this area, we conclude the present article with an overview of recently published theoretical studies on the mechanisms for framework atom removals from zeotype materials. We order the overview according to the material and the tetrahedral atom being removed. Zeolites: Desilication. In our recent publication57 on the desilication of SSZ-13, we present a mechanism involving a vicinal disilanol intermediate. Relative to a water molecule adsorbed on the silica wall, the energy of this intermediate is 111 kJ mol−1. A similar structure had earlier been proposed by Catlow and co-workers,58 and the energy (144 kJ mol−1) is in reasonable agreement. The rate-determining transition state of the desilication has a relative potential energy of 300 kJ mol−1. Zeolites: Dealumination. In the same work57 we also presented a mechanism of the dealumination of zeolite. This mechanism is similar to the desilication mechanism, but the presence of the Brønsted acid site makes the initial water adsorption more favorable. Furthermore, the rate-determining transition state is of lower relative energy (260 kJ mol−1). Very recently, Silaghi et al. published an alternative pathway for the first hydrolysis step of the dealumination of zeolites.59 In the pathway studied, water adsorbs on the aluminum atom instead of the Brønsted acid site, and the mechanism is reminiscent of pathways 2 and 3 of this work. Zeolite structures varying in both topology and in the location of the Al atom and the Brønsted acid site were investigated. The lowest activation energy (76 kJ mol−1) was determined for CHA with the Brønsted proton bonded to the O3 atom. The highest activation energy (120 kJ mol−1) was determined for MFI with Al at the T10 position and the Brønsted proton bonded to the O2 atom. Based on the computed data, a Brønsted−Evans− Polanyi (BEP) relationship was identified between the activation energy and the water dissociation energy. To avoid confusion and facilitate comparison between different studies, we would like to remark that for CHA water adsorption on the Brønsted proton is a more stable adsorption site. In this work we have determined the water adsorption enthalpy of H-SSZ-13 to be 100 kJ mol−1. SAPOs: Desilication. The desilication of SAPO-34 of this work involves a water adsorption on an aluminum atom. The mechanism shows therefore some similarities to the mechanism reported by Silaghi et al.59 The free energy span48 of the desilication is 145 kJ mol−1. SAPOs: Dephosphoration. In the companion article, we publish a mechanism of dephosphoration from SAPO-34. Also, the dephosphoration is initiated by a water molecule adsorbing on an Al atom. The free energy span48 of the dephosphoration is 189 kJ mol−1.54

Pathways 2 and 3 are significantly favored over pathway 1. In contrast to pathway 1, pathways 2 and 3 are characterized by water molecules coordinating to aluminum atoms. In this way pathways 2 and 3 are utilizing the Lewis acidity of aluminum in the hydrolysis of the T−O−T bonds. This is likely the cause of their much lower barriers compared to pathway 1. Furthermore, we find that the dispersion correction is necessary to compute the adsorption enthalpy of water molecules on zeolite materials acceptably close to the experimental value. Inclusion of the free energy correction is necessary to take into account the entropy loss upon adsorption of water molecules. Both dispersion and free energy corrections must be included in order to evaluate the concerted action of more than one water molecule. The free energy profile of the desilication shows that the process is endergonic. However, we suggest that healing of the hydrogarnet defect generated by the desilication might shift the equilibrium significantly toward the desilicated state and mobile Si species.

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ASSOCIATED CONTENT

* Supporting Information S

The cif files of all discussed intermediates; cif files of transition states for which phonon calculations have been carried out; NEB images provided as axsf files. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], phone +4798243934 (O.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This publication forms a part of the inGAP Center of Researchbased Innovation, which receives financial support from the Research Council of Norway under Contract No. 174893. The authors thank the Norwegian High Performance Computing program for a generous grant of computing resources. T.F. acknowledges a postdoctoral fellowship from the Research Council of Norway under the KOSK II program. The reviewers are thanked for their constructive feedback.

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REFERENCES

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CONCLUSIONS Modeling of the interactions of water/steam and microporous materials is an extensive task of formidable complexity. By exploring a substantial part of the potential energy landscape, we have compared three different mechanisms for the desilication of SAPO-34. The first mechanism, pathway 1, corresponds as closely as possible to the corresponding zeolite (SSZ-13) reaction, viz. dealumination. In pathway 2, water is allowed to coordinate to aluminum rather than to the acidic proton, thus yielding a smaller activation energy. Allowing an extra water molecule as a catalyst enhances the reaction further. 2083


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Desilication of SAPO-34: Reaction Mechanisms from Periodic DFT

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