Mechanistic Comparison of the Dealumination in SSZ-13 and the

Jun 24, 2013 - With the purpose of understanding the behavior of aluminosilicate zeolites and silicoaluminophosphates (SAPOs) in the presence of steam...
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Mechanistic Comparison of the Dealumination in SSZ-13 and the Desilication in SAPO-34 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, Department of Process Chemistry, P.O. Box 124 Blindern, 0314 Oslo, Norway S Supporting Information *

ABSTRACT: With the purpose of understanding the behavior of aluminosilicate zeolites and silicoaluminophosphates (SAPOs) in the presence of steam, we carried out a computational density functional theory (DFT) study on the desilication of SAPO-34. The mechanism studied was a stepwise hydrolysis of the four bonds to the Si heteroatom. An analogous process to the desilication of SAPO-34 is the dealumination of SSZ-13. To investigate possible mechanistic differences between the two processes, we compared the results of this study with the results of a previous study on dealumination in SSZ-13. We found that the intermediates along the dealumination path of SSZ-13 have one of the protons bonded to a bridging oxygen atom. In the corresponding intermediates of the desilication path in SAPO-34, the same proton prefers to be part of an aqua ligand coordinated to an Al atom. The principal factor determining the different proton locations is the electronic requirement of the atoms surrounding the proton. The different proton locations in SSZ-13 and SAPO-34 put clear conditions on possible mechanisms, thus causing them to be different for the two materials. We expect the principles determining the proton location also to be valid for other mechanisms of dealumination in SSZ-13 and desilication in SAPO-34.

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INTRODUCTION The methanol-to-olefin process (MTO)1 is an important transformation because it represents a flexible alternative toward olefin production from biomass, natural gas, and coal. The MTO process is catalyzed by zeotype materials such as the zeolite ZSM5 and the silicoaluminophosphate SAPO-34. Despite its success, the MTO process still faces some challenges concerning the relatively fast deactivation by coking. Fortunately, the coked catalyst can be regenerated by heating it in a flow of oxygen. In the process of burning off the coke (calcination), water is generated. Water is also generated as a byproduct of the MTO reaction. For each methanol molecule converted, one water molecule is produced. For these two reasons, the catalytic material is exposed to a considerable water vapor pressure. The presence of water affects the properties of the material. Interestingly, several reports indicate that water affects zeolites and silicoaluminophosphates (SAPOs) differently. In the case of zeolites, Campbell et al. found the presence of water to be beneficial for the MTO stability.2,3 For SAPOs, on the contrary, Aramburo et al. reported water to have a detrimental effect on the MTO stability.4 For both zeolites and SAPOs, the presence of water affects the heteroatoms of the material. (In this context heteroatom refers to the substitutional defect in the pure silica or AlPO material. In SSZ-13 the heteroatom is Al, and in SAPO-34 the heteroatom is Si.) For zeolites there are reports on an accumulation of aluminum on the external surface of the particles.2,5,6 On the contrary, for SAPOs several works report the isolated silicon atoms to cluster into silicon islands.7−10 © 2013 American Chemical Society

Lastly, steaming tends to cause generation of mesopores in zeolites,5,11,12 while for SAPOs one work reports that mesopores do not form.4 In light of the structural similarities between zeolites and SAPOs, their different behavior in the presence of steam is striking. It would therefore be of fundamental interest to understand the cause of these differences. Such an understanding could perhaps later pave the way for design of better catalysts with higher MTO stability. A first step toward this goal is a better understanding of the atomistic level of how steam modifies zeolites and SAPOs. In particular it would be of vital interest to know if the differences observed at the macroscopic level are reflected in mechanistic differences at the atomistic level. Computational methods such as the density functional theory (DFT) are suited for this task. By gradually modifying the atomic coordinates, we can monitor the potential energy change from one intermediate to the next one. We recently published a DFT study on the dealumination of the zeolite SSZ-13 by hydrolysis.13 An analogous process to the dealumination of SSZ-13 is the desilication of SAPO-34. SSZ-13 and SAPO-34 are both of the chabazite topology, and in both processes it is the heteroatom that is being removed from the framework. The clear analogy between the two systems makes it easier to assess their mechanistic differences. As a natural starting Received: March 22, 2013 Revised: May 31, 2013 Published: June 24, 2013 13442

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components of the forces were below 1.4 × 10−2 eV au−1, and the energy did not change by more than 1.4 × 10−3 eV between iterations. For the variable cell optimizations, the convergence criterion for the pressure was set to 0.5 kBar. The reaction pathways between the intermediates were described by nudged elastic band (NEB) calculations22,23 with ten images including the start and end points.

point, we have investigated whether the intermediates and reaction barriers previously published for the dealumination of SSZ-13 are also feasible for the desilication of SAPO-34.

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METHODS Model of the Catalyst. The introduction of the heteroatoms in the pure silicalite chabazite and AlPO-34 causes the materials to be negatively charged. This negative charge is compensated by the introduction of a cation in the vicinity of the heteroatom. Several cations 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 heteroatom. For both SSZ-13 and SAPO-34 the literature is ambiguous with respect to which location is the most stable.14−17 In ref 13 and in this work we have placed the proton at O(2) (see Figure 1 for an explanation of the labeling system).

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RESULTS The potential energy profile of the dealumination of SSZ-13 reproduced from ref 13 is shown in Figure 2. The mechanism is characterized by a stepwise hydrolysis of the four Al−O−Si bonds. Each Al−O−Si breakage is assisted by the action of an adsorbed water molecule. We label the SSZ-13 intermediates in Figure 2 as Z0, ..., Z5 in their consecutive order in the reaction path (Z for zeolite). For the structures with an adsorbed water molecule, we added a -H2O to the respective label. The SSZ-13 transition states are labeled Z-TS1, ..., Z-TS5 in their consecutive order in the reaction path. To check the mechanistic similarity between the SSZ-13 dealumination and the SAPO-34 desilication, we optimized the SAPO-34 intermediates from the geometries of the SSZ-13 intermediates. We labeled the SAPO-34 intermediates S0, ..., S5 (S for SAPO) according to the SSZ-13 intermediates from which they were optimized. In the same manner as for the SSZ-13 intermediates, we added a -H2O to the label of structures with a water molecule adsorbed. For SAPO-34 intermediates with more than one conformer, we added a lowercase letter to the label. Similarly as for SSZ-13, we labeled the SAPO-34 transition states as S-TS1, ..., S-TS7 in their consecutive order along the reaction path. Our initial hypothesis was that the SAPO-34 intermediates would be qualitatively similar to the SSZ-13 geometries from which the optimizations were started. It turned out, however, that most of the SAPO-34 intermediates were radically different from the SSZ-13 intermediates. Only the structures corresponding to the hydrolysis of the first T−O−T bond were qualitatively similar in the two materials. For SAPO-34 structures optimized from structures later than Z2 on the reaction path, significant geometry changes occurred during the optimizations. The intermediates after Z2 on the SSZ-13 path are all characterized by having a proton bonded to an oxygen atom bridging two tetrahedrally coordinated atoms (Al and Si). During the optimization of the SAPO-34 structures, this proton is transferred over to a nearby Al−OH group. From the radically different geometries of the SAPO-34 intermediates compared to the SSZ-13 intermediates, it is evident that the two mechanisms are qualitatively different. We computed the potential energy profile of the desilication of SAPO-34 (see Figure 3) by connecting the optimized intermediates through NEB calculations. In the following paragraphs we show how the mechanism of the SAPO-34 pathway differs from the mechanism of the SSZ-13 pathway. Hydrolysis of the First Bond. From intermediates Z0H2O/S0-H2O to intermediates Z2/S2, the mechanisms of the two processes are qualitatively similar. Both the SSZ-13 mechanism and the SAPO-34 mechanism go through a vicinal disilanol intermediate,24,25 Z1/S1, before ending up in Z2/S2 with the coordination sphere of the heteroatom inverted. The structures of these intermediates are shown in Figure 5. Schematically, the first hydrolysis step can be described as follows (see Figure 4)

Figure 1. 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.18 The aluminum atoms are labeled according to the oxygen atom to which they are connected. For SSZ-13 the labeling scheme is analogous, with Al replaced by Si.

In line with the close analogy between the two systems, we have chosen the same zero level of the energy scale for the desilication of SAPO-34 as for the dealumination of SSZ-13, i.e., the pure material without any adsorbed water. Computational Details. Periodic density functional calculations were performed using the Quantum Espresso computational code.19 To model the SAPO-34 material, the orthorhombic unit cell of chabazite (12 T-atoms, 37 atoms in total) was used. The functional was PBE.20 The core electrons were replaced by ultrasoft pseudopotentials,21 and the valence electrons were expanded in a plane-wave basis set with a kinetic energy cutoff of 680 eV. The Brillouin-zone sampling was restricted to the Γ-point. The convergence criterion of the SCF calculations was set to an energy difference of 1.4 × 10−5 eV. For the pure SAPO-34 material with no water adsorbed, both the unit cell parameters and the atomic coordinates were optimized. For the rest of the structures, the optimized unit cell parameters of the pure material were fixed, and the atomic coordinates were optimized. The structures were considered optimized when all 13443

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Figure 2. Dealumination pathway of the SSZ-13 zeolite. Potential energy profile in kJ mol−1. Some atoms are colored to indicate their origin at the start of the reaction. Z0-H2O is a more stable conformer of the corresponding structure in ref 13.

(1) The proton of the adsorbed water molecule (cyan color in Figure 6) is transferred over to O(1), and the remaining OH group coordinates to Si1. 2) The O1aqH group (red color in Figure 6) breaks its bond to Si1 and forms a bond to Al. (3) The Si4−O(4)H group breaks its bond to Al. Starting from the geometry of Z2-H2O, the structure of S2aH2O (−80 kJ mol−1) is optimized through a chained proton transfer from O(4) via the adsorbed water molecule to O1aq (see Figure 8). Similarly, structure S3a (−11 kJ mol−1) is optimized from the geometry of Z3 through a chained proton transfer from O(1) via O1aq to O2aq. The two SAPO-34 intermediates, S2aH2O and S3a, are qualitatively different from the corresponding SSZ-13 intermediates. Therefore, it seems unlikely to find a reaction path similar to the corresponding SSZ-13 path shown in Figure 6. We therefore connected structure S2a-H2O and S3a through an alternative pathway, which schematically can be described as follows (see Figure 7): (1) The adsorbed water molecule transfers its proton to O(4). Simultaneously, a proton of the aqua ligand at Al1 is transferred to the remaining OH group of the water molecule. (2) The regenerated water molecule (cyan color in Figure 7) is coordinated to Al4, while the Si−O(4)H group is decoordinated. During the NEB optimization of this pathway, two lower energy conformers (S2b-H2O (−82 kJ mol−1) and S3b (−50 kJ mol−1)) of S2a-H2O and S3a were located. In the SAPO-34 pathway

(1) From Z0-H2O/S0-H2O one proton of the adsorbed water molecule is transferred over to O(4), while the HO1aq group binds to Si4/Al4 and the heteroatom. The system then relaxes to the vicinal disilanol intermediate, Z1/S1. (2) From Z1/S1 the HO1aq group breaks its bond to Si4/Al4 and forms a bond to Si1/Al1. (3) O(1) breaks its bond to Si1/Al1 and forms a bond to Si2/ Al2 (4) The HO(2) group breaks its bond to Si2/Al2, and the system relaxes to Z2/S2. Contrasting the geometric similarities between SSZ-13 and SAPO-34 along the first hydrolysis step is the difference in the barrier of the second elementary step. Z-TS2 has an energy of 195 kJ mol−1, while S-TS2 has an energy of merely 119 kJ mol−1. However, the geometries of these two transition states are qualitatively similar; the reaction paths have not diverged yet. The significant difference in their energies may therefore be explained by the different bonds being broken. In Z-TS2 a strong Si−O bond breaks (bond dissociation energy (BDE): 800 kJ mol−126), while in S-TS2 a weaker Al−O bond is being broken (BDE 511 kJ mol−126). Hydrolysis of the Second Bond. The intermediates Z2H2O and S2b-H2O preceding the second hydrolysis step mark the point along the dealumination/desilication pathway where the two mechanisms start to diverge. The part of the SSZ-13 pathway connecting Z2-H2O and Z3 can be described schematically as follows (see Figure 6): 13444

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Figure 3. Potential energy profile of the SAPO-34 desilication pathway. Energies in kJ mol−1. Some atoms are colored to indicate their origin at the start of the reaction.

similar (see Figure 8) and differ only by 2 kJ mol−1. Structure S3b and structure S3a, on the other hand, differ significantly from each other. In structure S3b the Si−O(4)H group is coordinated strongly to Al4. The coordination sphere of Al4 is therefore trigonal bipyramidal as opposed to tetrahedral in structure S3a. This structural difference is presumably the cause of the lower energy of S3b. The reaction barrier of the second hydrolysis is defined as the difference between the transition state, S-TS4 (2 kJ mol−1), and the preceding intermediate, S2b-H2O (−82 kJ mol−1). Its height of 84 kJ mol−1 is significantly lower than the corresponding reaction barrier for SSZ-13 (191 kJ mol−1). Hydrolysis of the Third Bond. The SAPO-34 intermediate preceding the third hydrolysis step, S3b-H2O (−91 kJ mol−1), was obtained by adsorption of a water molecule to the HO(4) group of structure S3b. The intermediate succeeding the reaction step, S4 (−54 kJ mol−1), was obtained through an optimization from the Z4 geometry. Following the general trend, during the optimization of S4, a proton transfer from O(1) to O3aq occurred (see structures in Figure 11). The geometric differences of the intermediates of this hydrolysis step (Z3-H2O/S3b-H2O and Z4/S4) determine the mechanistic differences between the SSZ13 pathway and the SAPO-34 pathway. The SSZ-13 reaction step (see Figure 9) can schematically be described as an insertion of the adsorbed water molecule (purple color in Figure 9) into the Al−O(3) bond. The SAPO-34 reaction step (see Figure 10) is described schematically in two steps as follows:

Figure 4. Schematic representation of the first hydrolysis step. The SSZ13 case is shown here, but the SAPO-34 case is completely analogous.

(1) A coordination of the water molecule (purple color in Figure 10) to Al3.

(Figure 3) the most stable conformers, S2b-H2O and S3b, are reported. The structures S2b-H2O and S2a-H2O are qualitatively 13445

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Figure 5. Intermediates of the hydrolysis of the first T−O−T bond for the zeolite (left) and the SAPO (right). 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 adsorbed water molecules are labeled Onaq where n = 1, ..., 4.

Figure 6. Schematic representation of the second hydrolysis step of the SSZ-13 pathway. The pathway connects Z2-H2O and Z3. Figure 7. Schematic representation of the second hydrolysis step of the SAPO-34 pathway. The path connects S2a/b-H2O and S3a/b.

(2) A breakage of the Al3−O(3) bond through a proton transfer to O(3). The barrier of the SAPO-34 reaction step is 147 kJ mol−1. This is considerably higher than the corresponding reaction barrier for the SSZ-13 pathway (89 kJ mol−1). Hydrolysis of the Fourth Bond. The last hydrolysis connects the intermediates Z4-H2O/S4b-H2O and Z5/S5b of the SSZ-13 and the SAPO-34 systems. The SSZ-13 reaction path

connecting Z4-H2O and Z5 can be described as an SN2 pathway (see schematic representation in Figure 12). The adsorbed water molecule is coordinated to the Al atom, while the Si−OH group leaves the coordination sphere of Al. In this process the coordination sphere of Al is inverted through a trigonal planar transition state. All bonds between the aluminum complex of Z5 13446

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Figure 8. Intermediates of the hydrolysis of the second T−O−T bond. S2a-H2O and S3a are optimized from Z2-H2O and Z3, respectively. S2b-H2O and S3b are located during the NEB optimization.

Figure 9. Schematic representation of the hydrolysis of the third Al−O− Si bond of the SSZ-13 system.

(Al(OH)3H2O) and the rest of the framework have been broken, and this complex must be considered an extra-framework species. Starting from the geometry of Z4-H2O, the S4a-H2O (−91 kJ mol−1) was optimized. During the optimization a proton transfer took place from O(1) to the OH group at Al3 (see structures in Figure 14). A more stable conformer of S4a-H2O, S4b-H2O (−113 kJ mol−1, Figure 14), with one more H-bond between the water molecule and the framework was also optimized. For S5a (−75 kJ mol−1) succeeding the hydrolysis step, the structure was optimized from the geometry of Z5. During this optimization, the proton of the aqua ligand at Si is transferred to (the mirror image of) O2aq. The proton originally located at O2aq is then

Figure 10. Schematic representation of the third hydrolysis step of the SAPO-34 pathway.

transferred to the OH group at Al2. In the structure of S5a the extra-framework species is silicic acid Si(OH)4. This puts clear 13447

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Figure 11. Intermediates of the third hydrolysis step of the SSZ-13 and the SAPO-34 pathways. The atoms are colored according to their elements: red = O, green = P, white = H, yellow = Si, and magenta = Al.

Figure 13. Mechanism of the last hydrolysis step for the SAPO-34 desilication pathway.

Figure 12. Schematic representation of the last hydrolysis step in the SSZ-13 pathway. This reaction step follows an SN2 type mechanism with H2O as the entering group and HO-Si as the leaving group.

the water molecule (green color in Figure 13) coordinates to Al2, while one of its protons transfers to the OH group at Al4. Thereafter, the proton originally at the Al4-OH group (cyan color in Figure 13) transfers to O(1), while the Al2−O(1) bond breaks. The system then relaxes to structure S5c. The second step consists of a conformational change toward the more stable conformer S5b.

constraints on the mechanism of the last hydrolysis step of the SAPO-34 pathway. Here, an SN2 type mechanism like that of the corresponding SSZ-13 reaction step is inconceivable. A schematic representation of the last hydrolysis step of the SAPO-34 pathway is shown in Figure 13. In the optimization of this pathway, two new conformers of S5a, S5b (−93 kJ mol−1) and S5c (−75 kJ mol−1), were discovered. This pathway consists of two elementary steps. In the first step, from S4b-H2O to S5c, 13448

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Figure 14. Initial and final intermediates of the last hydrolysis step of SAPO-34 and SSZ-13. S4a-H2O and S5a are optimized from the geometries of Z4H2O and Z5, respectively. S4b-H2O and S5b are the most stable conformers reported in the potential energy profile of Figure 3.

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DISCUSSION

A possible explanation for the different structural preferences may be found by considering the pure silica chabazite and AlPO34 materials. In pure silica, all Si atoms are of oxidation state +4 (the electrons are formally assigned to the more electronegative oxygen atoms) and formal charge zero (four valence electrons and eight electrons shared in covalent bonds). When a Si atom is substituted by an Al atom and a proton to form SSZ-13, the electronic preference of the T atoms is largely the same. In all SSZ-13 intermediates from Z2-H2O to Z5, the oxidation state of the Al heteroatom is +3 (the electrons are formally assigned to the surrounding O−Si and/or OH groups), while the formal charge is zero (three valence electrons and six electrons shared in covalent bonds). In the same intermediates, the Si atoms involved in the hydrolysis of the T−O−T bonds have an oxidation state of +4 and a formal charge of zero. The surrounding groups of these Si atoms are three O−Si groups and one group that can be either O−Al, OH terminal, or OH Brønsted. For both the Al and the Si atoms to have a formal charge of zero, the Brønsted proton must stay at the bridging

The results of this work provide important mechanistic insight into the dealumination of SSZ-13 and the desilication of SAPO34. During the hydrolysis of the first T−O−T bond, the mechanisms of the two processes are qualitatively similar. However, once the first T−O−T bond is broken, the two mechanisms diverge. From Z2-H2O/S2-H2O and onward, the SSZ-13 and SAPO-34 intermediates corresponding to the same state of hydrolysis show significant structural differences. The pairs of structures Z2-H2O/S2-H2O, Z3/S3, Z3-H2O/S3bH2O, Z4/S4, Z4-H2O/S4-H2O, and Z5/S5 are distinguished by the location of a proton. In the SSZ-13 structures this proton is a Brønsted proton bonded to an oxygen atom bridging Al and Si. In the corresponding SAPO-34 structures, the Brønsted proton is transferred over to a nearby OH group. The different structural preferences of SSZ-13 and SAPO-34 are the cause of the different mechanisms for dealumination of SSZ-13 and desilication of SAPO-34. 13449

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account in future mechanistic studies on hydrolysis of zeolites and SAPOs. Furthermore, the work may have important implications for the study of more complex phenomena in zeolites and SAPOs related to the presence of steam. Such phenomena might be mesopore generation in zeolites and Si island formation in SAPOs. The MTO process is catalyzed by zeolites and SAPOs, and an insight into how the catalytic material changes under operational conditions is vital for the optimization of the process with respect to parameters such as catalyst stability, activity, and product selectivity. We therefore believe that this study is a first step toward that goal.

oxygen atom. If it were to move to a nearby silanol group, the Al atom would get a formal charge of −1, and the Si atom with the aqua ligand would get a formal charge of +1. That would lead to a less stable structure. In a pure AlPO-34 structure, the formal charge of the phosphorus atoms is +1, and the formal charge of the aluminum atom is −1. When a phosphorus atom is substituted by a silicon atom and a proton to form SAPO-34, the silicon atom obtains a formal charge of zero. The Al atom bonded to the Brønsted site obtains a formal charge of zero, while the other surrounding Al atoms maintain their charge of −1 as in the AlPO-34 material. In the intermediates from S2-H2O to S5, the silicon atom prefers a formal charge of zero, while the Al atoms connected to Si through a bridging oxygen atom prefer a formal charge of −1. These two requirements cannot be fulfilled with the Brønsted proton bonded to the bridging oxygen atom. Therefore, the proton prefers to be bonded to one of the nearby HO-Al groups thus forming an aqua ligand bonded to an Al atom. In this way one of the Al atoms whose connection to the Si heteroatom is broken obtains a formal charge of zero. The other Al atoms affected by the hydrolysis have a terminal OH ligand and thus a formal charge of −1. To sum up, in SSZ-13 the proton is bonded to a bridging oxygen atom. This location is determined by the requirement that both the Al atom and the surrounding Si atoms must have a formal charge of zero. In SAPO-34 the proton is part of the aqua ligand coordinated to an Al atom. This location is determined by two charge requirements: The Si atom must have a formal charge of zero, and the Al atoms connected to Si must have a formal charge of −1. Contrasting the mechanistic discrepancy from Z2-H2O/S2H2O and toward the end of the reaction is the similarity between the two mechanisms along the hydrolysis of the first bond. Taking into account the large discrepancy later on along the reaction path, the mechanistic similarity along the hydrolysis of the first bond is striking. A possible explanation for this similarity in the first part of the reaction might be the constrained geometric environment around the heteroatom. The pure SSZ13 and SAPO-34 starting materials, Z0 and S0, are qualitatively similar. Before the breakage of the first T−O−T bond, there is not much geometrical degree of freedom, and the two systems are forced to follow the same mechanism. Very clearly, the energetically topmost point of the reaction pathway for desilication of SAPO-34 (S-TS1 +118 kJ mol−1 or STS2 +119 kJ mol−1) is substantially lower than the highest point on the pathway for the dealumination of SSZ-13 (Z-TS2 +195 kJ mol−1). Also, the highest barriers appear early in the reaction scheme, before the pathways diverge. This might be interpreted to indicate that SAPO-34 could be more susceptible to framework degradation during steaming than the zeolite analogue, which is at least in partial agreement with relevant experimental reports. However, we are reluctant to draw definitive conclusions on the matter, given that we have only explored a very limited part of the potential energy surface for the two processes. The limited potential energy exploration will, however, not affect the main findings of this work: that the two processes are fundamentally different. The principle guiding the preferred location for the proton in SSZ-13 and SAPO-34 is likely to be valid also for other mechanisms of dealumination of SSZ-13 and desilication of SAPO-34. The present results should therefore be taken into

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CONCLUSIONS We have compared mechanisms of dealumination in SSZ-13 and desilication in SAPO-34. Along the hydrolysis of the first T−O− T bond, the two mechanisms are qualitatively the same. After the breakage of the first T−O−T bond, however, the mechanisms of the two processes start to diverge. The main differences are related to the preferred location of a particular proton. In SSZ-13 this proton is a Brønsted proton bonded to an oxygen atom bridging Al and Si. This location is a consequence of the charges of the Al heteroatom and the surrounding Si atoms. Both the Al and the Si atoms have a preference for a formal charge of zero. If the Brønsted proton is removed from the bridging oxygen atom, the formal charge of the Al atom will be −1, and there will be a charge of +1 somewhere else in the material. In SAPO-34 the proton prefers to be part of an aqua ligand coordinated to an Al atom in the vicinity of the Si heteroatom. Again, this location is a consequence of the formal charges on the Si heteroatom and the surrounding Al atoms. The Si heteroatom prefers a formal charge of zero, and the Al atoms not affected by the hydrolysis prefer a formal charge of −1. With this charge requirement the proton cannot be bonded to a bridging oxygen atom. The location of the proton constrains the mechanistic possibilities in the two systems and is the main cause of the mechanistic differences. The proton location is related to fundamental properties of the materials. We therefore expect the findings of this work also to be valid for other dealumination/ desilication mechanisms.

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ASSOCIATED CONTENT

S Supporting Information *

CIF files of all discussed intermediates and .axsf files of the NEB images of the reaction pathways. 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. 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 Norwegian High Performance Computing program (http:// www.notur.no) is thanked for a generous grant of computing resources. T. F. acknowledges a postdoctoral fellowship from the Research Council of Norway under the KOSK II program. 13450

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