Mechanism of Si Island Formation in SAPO-34 - The Journal of

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Mechanism of Si Island Formation 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, P.O. Box 124 Blindern, 0314 Oslo, Norway S Supporting Information *

ABSTRACT: With the aim of understanding the Si island formation in SAPO-34, we have carried out a computational mechanistic study. Briefly, the Si island formation in SAPO-34 is explained by three successive reactions. First, the framework Si atom is removed from the framework through the action of four water molecules. Second, the hydrogarnet defect generated by the desilication is healed by an available H3PO4 molecule. Third, the extra framework Si(OH)4 species inserts in the framework position of a phosphorus atom while, in a concerted fashion, “kicking out” the phosphorus atom as a H3PO4 extra-framework species. When these exchanges of framework and extra-framework species are repeated, the isolated Si atoms may eventually cluster into Si islands.

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INTRODUCTION Catalyst deactivation is a major problem in many industrial processes, including the methanol-to-olefin (MTO) process.1−3 The MTO process is catalyzed by microporous materials such as ZSM-5 and SAPO-34. These catalysts are deactivated through the formation of unreactive aromatic carbon species (“coke”), which may block active sites or hamper the diffusion of reactants/products by blocking the micropores of the materials. The catalysts are then regenerated by heating them in air, burning off the coke. The catalyst may be regenerated many times, but eventually it will suffer from permanent activity loss.4,5 For SAPO-34, this is likely caused by the formation of Si islands with the concomitant loss of Brønsted active sites.6−9 Because the activity loss is permanent, the process is often called “irreversible deactivation”. Si island formation requires that Si atoms and P atoms in the original SAPO-34 material are mobilized and exchange sites. The mobilization is likely caused by hydrolysis of T−O−T bonds (T = tetrahedrally coordinated atoms) by available water molecules. Under MTO conditions, water molecules may originate from three main sources: (1) water is generated as a byproduct of the MTO reaction, (2) water is produced in the burnoff of the coke species in an oxygen-containing atmosphere, and (3) water is unavoidably present in the regeneration gas. Apart from this superficial knowledge, little is known about the details of silicon island formation. A first step toward inhibiting the irreversible deactivation of the SAPO-34 catalyst is to acquire a detailed knowledge of the mechanism of Si island formation. In this work we propose a mechanism for the initial steps of the Si island formation. Briefly, the proposed mechanism consists of the following consecutive processes: (i) initial desilication by four water molecules (presented in the companion article10), (ii) insertion of a H3PO4 molecule into © 2015 American Chemical Society

the generated defect, and (iii) exchange of Si and P framework atoms.

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METHODS Model of the Catalyst. The substitution of a phosphorus atom by a silicon atom in pure AlPO-34 causes the material to be negatively charged. In H-SAPO-34 this negative charge is compensated by the introduction of a proton in the vicinity of the Si atom. 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.11−14 In our previous publications15 and in this work we have placed the proton at O(2) (see Figure 1 for an explanation of the labeling system). Computational Details. Periodic density functional calculations were performed using the Quantum Espresso code.17 Two reactions were explored in this work. For the healing of the hydrogarnet defect, the orthorhombic unit cell (36 atoms) was used. For the Si/P exchange, the hexagonal unit cell (108 atoms) was used. The potential energy of the structures was computed using the approach described in our earlier work.15 In this work, however, we used the Grimme dispersion correction18,19 throughout. The reaction pathways between the intermediates were described by nudged elastic band (NEB) calculations20,21 with 10 images including the start and end 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 (CINEB).22 Received: October 29, 2014 Revised: December 18, 2014 Published: January 20, 2015 2086

DOI: 10.1021/jp510845z J. Phys. Chem. C 2015, 119, 2086−2095

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The Journal of Physical Chemistry C

material (structure 20, see below). To exchange with a phosphorus species, Si(OH)4 adsorbs at a location close to the phosphorus species to be exchanged (structure 21, below). In this adsorption, the translational and rotational degrees of freedom (DOF) of Si(OH)4 transform into vibrational DOF. For structure 20, we computed the vibrational modes in the orthorhombic unit cell. Six of the modes correspond to rotations and translations of the Si(OH)4 species. The change in the free energy contribution from 20 to 21 we approximated as the difference between treating these modes as vibrations and treating them as pure translations and rotations. This change in the free energy contribution was then added to the potential energy change to compute the total free energy change from 20 to 21. For the rest of the Si/P exchange, we assumed the free energy contribution to be constant. In the companion article,10 we show that as long as no adsorption/ desorption occurs during the relevant steps, this is a good approximation. Modeling of the Reaction Conditions. Of particular relevance to this contribution is the work of Vomsheid et al.,23 who studied the Si island formation in SAPO-34 materials at room temperature in humid air. On the other hand, because of the industrial implications, we are primarily interested in understanding the Si island formation at MTO conditions (623−873 K and water vapor pressure between 1 and 6 atm24). However, the high temperature poses some modeling challenges. One problem concerns the vibrational contribution to the free energy. At 623 K a number of vibrational modes are excited, some of them by several quanta. Furthermore, the harmonic approximation loses validity as temperatures increase. A second problem is that at high temperatures highly activated reaction pathways become significant, potentially leading to the reaction following several different mechanisms at the same time. However, as our main aim is to demonstrate that pathways of reasonably low activation energies are available for the proposed mechanisms, this issue may be disregarded. Third, at very high temperatures, energy transfer by radiation becomes non-negligible. Thus, as a compromise, we have opted for modeling the Si island formation by reporting free energies at the conditions of ref 23 (298 K and 0.02 atm water vapor pressure). These results we extrapolated to higher temperature and water pressure by carrying out microkinetic modeling using the CatMap program.25 As energy values, we provided the free energies at a given temperature. To study the temperature dependence of the rate, we carried out separate simulations at specific temperatures.

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.16 The aluminum atoms are labeled according to the oxygen atom to which they are connected.

For the reaction calculated with the orthorhombic unit cell, the vibrational Gibbs free energy correction was added to the potential energy for stationary points along the reaction path. This was done by use of phonon calculations. For the free water molecules, the vibrational contribution to the Gibbs free energy correction was obtained by a phonon calculation. The rotational and translational contributions were calculated from the stoichiometry and geometry of the molecule. For the calculation of the pressure volume work, the ideal gas approximation, PV = RT, was used. Further details on the free energy calculation are described in the companion article.10 For the Si/P exchange reaction path, the use of the orthorhombic unit cell led to significant and unphysical interactions between the mirror images for certain intermediates. To remove these artifacts, the hexagonal unit cell (108 atoms) was used. We deemed it too expensive to carry out phonon calculations on the hexagonal unit cell, but we approximated the free energy curve of the Si/P exchange by considering the free energy change of the Si(OH)4 species at the start of the process. The Si/P exchange initiates with Si(OH)4 moving freely in the micropores of the SAPO-34

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RESULTS Insertion of H3PO4 into the Hydrogarnet Defect. We have proposed10 the desilication of SAPO-34 to be the first step of the Si island formation. The desilication is characterized by a reaction free energy of +94 kJ mol−1. Furthermore, the energy

Figure 2. Schematic representation of the healing process of the hydrogarnet defect. 2087


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Figure 3. Free energy profile (kJ mol−1) of the H3PO4 insertion into the hydrogarnet defect computed at T = 298 K and PH2O = 0.02 atm. Some atoms are colored according to the water molecules to which they initially belonged.

span26 of the forward reaction is 145 kJ mol−1. The barrier of the first irreversible step of the reverse reaction is 51 kJ mol−1. The high reaction free energy causes the equilibrium to be largely shifted toward silicon being in the framework position. The small degree of desilication is seemingly in contradiction with the experimental observation of Si island formation.6,7,23,27,28 The contradiction can, however, be explained by realizing that the desilication process10 has been described for a simplified model system, consisting of a purely crystalline SAPO-34 material with necessary water molecules adsorbed. The real SAPO-34 material contains a significant density of different defects. In particular, the existence of extra-framework H3PO4 species is possible (see below). H3PO4 competes with Si(OH)4 for the insertion into the hydrogarnet defect. Upon insertion into the hydrogarnet defect, the H3PO4 molecule is transformed into a framework phosphorus atom while four water molecules are released (see schematic representation in Figure 2). In the following we describe the main features of the H3PO4 insertion mechanism. Details are given in the Supporting Information. Figure 3 shows the computed free energy profile of the H3PO4 insertion into the hydrogarnet defect. The starting point (and zero level) of the reaction path is defined as H3PO4 coordinated to Al completely separated from the hydrogarnet defect (see Figure 4). The first part of the reaction pathway, from 0 (Figure 4) to 10 (Figure 5), is characterized by two proton transfers from the H3PO4 species to the OH groups of the hydrogarnet defect. The OH groups are converted to water molecules, and O−Al bonds are formed between the original H3PO4 species and the hydrogarnet defect. Figure 5 shows a 3D representation of 10. The OH group of the inserting PO4 moiety is pointing away from the OH group of Al3. In the last part of the H3PO4 insertion, the coordination sphere of the PO4 moiety rotates such that the P−OH group can approach the Al−OH group. The POH group then reacts

Figure 4. 3D structures showing the starting point of the H3PO4 insertion (0, 0 kJ mol−1): (A) the H3PO4 extra-framework species coordinated to an Al atom; (B) the hydrogarnet defect into which H3PO4 will enter.

with the Al−OH group to form a water molecule and a Al−O− P bond. The free energy profile in Figure 3 reveals several thermodymanic and kinetic characteristics of the H3PO4 insertion. The H3PO4 insertion is highly irreversible with a reaction free energy of −163 kJ mol−1. Furthermore, the free 2088


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as 7 kJ mol−1. We use this value as a lower limit of the uncertainty of the difference between the barrier heights we are considering. Because the difference in the barrier heights is only 9 kJ mol−1, we are not able to determine whether H3PO4 or Si(OH)4 inserts preferably into the hydrogarnet defect. In contrast to the reaction barriers, the reaction energies of the two insertions are significantly different. For the H3PO4 insertion, the reaction energy is −163 kJ mol−1, and for the Si(OH)4 insertion, it is −94 kJ mol−1. Even though H3PO4 and Si(OH)4 insert into the hydrogarnet defect at a comparable rate, H3PO4 is essentially trapped after being converted to a framework phosphorus atom, while Si(OH)4 has a significantly higher probability of leaving the framework position. We therefore conclude that because of the irreversibility of the insertion of H3PO4, the micropores are gradually populated with extra-framework Si(OH)4 species that diffuse to other locations of the material. Extra-framework species have been observed in zeotype materials.30,31 The assumption of the presence of the H3PO4 species should therefore not be very risky. However, the concentration of such species may vary significantly depending on how the material was synthesized and on the reaction conditions. Because we wish to model a phenomenon as generally as possible, we should also take into account a very low H3PO4 concentration. Therefore, we will consider different processes by which the H3PO4 species can be generated. H3PO4 Generation from a Framework P Atom. The concentration of framework P atoms is very high. One might therefore be tempted to suggest that the extra-framework H3PO4 species can be generated through hydrolysis from a framework P atom. However, from the discussion above, this process is thermodynamically unfavorable; it is the reverse reaction as shown in Figure 3 and therefore unlikely to be of any consequence. H3PO4 Generation from a P Atom in a Defect Site. Zeotype materials contain defects; grain intergrowth boundaries and external surfaces are but two examples. P atoms present in such defects may have their coordination sphere perturbed in the direction of a lower coordination number. As a consequence, these P atoms are connected to one or more terminal OH group(s). P−OH species are readily observed in SAPO materials using FT-IR.32 Hydrolyzing such P atoms requires fewer than four water molecules. These structural characteristics should make the P atoms present in defect sites significantly easier to hydrolyze compared to framework P atoms. The detailed atomic structure of intergrowth boundaries and external surfaces is not well-known. However, structure 10 (Figure 5) may be considered as an approximation of a P atom present at the external surface or in a grain intergrowth

Figure 5. 3D representation of 10 (−118 kJ mol−1). The POH group is located away from the OH group of Al3.

energy difference between the rate-determining transition state (TS16−17, −13 kJ mol−1) and the rate-determining intermediate (10, −118 kJ mol−1) is 105 kJ mol−1. This makes the H3PO4 insertion a fast reaction at room temperature. Si(OH)4 Insertion versus H3PO4 Insertion. We will now assess the relative probability of Si(OH)4 insertion versus H3PO4 insertion. In such an assessment, the difference between the selectivity determining transition state and intermediate of the two processes has to be compared. For the H3PO4 insertion, the selectivities determining transition state and intermediate are TS4−5 (26 kJ mol−1) and 3 (−34 kJ mol−1), respectively. This difference (60 kJ mol−1) is to be compared with the corresponding difference for the Si(OH)4 insertion (51 kJ mol−1).10 When assessing the selectivity between the H3PO4 insertion and the Si(OH)4 insertion, the accuracy of the applied method should be taken into account. We base our error estimate on the benchmark study by Goerigk and Grimme.29 In this study, however, we compute the weighted total mean absolute deviation (WTMAD) of PBE-D3 for the subsets BHPERI and BH76 (33 kJ mol−1). We use this value as a rough accuracy measure for barrier heights calculated with the PBE-D functional. Both reactions whose barriers we want to compare consist of formation and breakage of O−Al and O−H bonds. Much of the uncertainty in the barrier heights should therefore cancel out. As a limiting case we might assume a complete error cancelation. In that case, the remaining error is likely to originate mainly from the noncovalent interactions. Goerigk et al.29 give the WTMAD of PBE-D3 for noncovalent interactions

Figure 6. Schematic representation of the Si/P exchange. 2089


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Figure 7. Free energy profile (kJ mol−1) of the Si/P exchange.

The substitution of a framework P atom with a Si framework atom is an endergonic process. The change in free energy from 20 to 38 is +53 kJ mol−1. The Si/P exchange is further characterized by the following structural changes: In the first part of the reaction path, the framework P atom inverts its coordination sphere. This inversion is made possible through the action of two water molecules. These water molecules assist the proton transfer from the Si(OH)4 species to the PO4 moiety. The inversion of the PO4 coordination sphere is necessary for Si(OH)4 to form bonds with the Al atoms that originally were bonded to PO4. In the next few elementary steps (see Figure 7) two proton transfers take place from the SiO4 moiety to the PO4 moiety. Simultaneously to these proton transfers, the Si−O groups form bonds to two more Al atoms originally bond to PO4. After these Si−O−Al bond formations, the SiO4 moiety is bonded to three Al atoms while the PO4 moiety is bonded to one Al atom. For the Si atom to be transformed into a tetrahedrally coordinated framework atom, an inversion of its coordination sphere must take place. The inversion of the SiO4 coordination sphere is catalyzed both by the available water molecules and the H3PO4 species. After this inversion the Si/P exchange is complete. Further details of the Si/P exchange are given in the Supporting Information. Structure 38 represents the final stage of the Si/P exchange, the free energy profile of which is shown in Figure 7. The barriers associated with the inversion of the PO4 and SiO4 coordination spheres are the highest barriers along the profile,

boundary. The free energy change from 10 to the fully hydrolyzed H3PO4 species, 0, is +118 kJ mol−1. Furthermore, the free energy barrier of the process is 144 kJ mol−1. Taking into account the approximations of the model, we estimate this barrier to be the upper limit of a dephosphorization from an external surface/grain boundary. The barrier height is reasonable for a slow reaction at room temperature. Once the H3PO4 species has been liberated, it may insert into a hydrogarnet defect according to the process in Figure 3. Mechanism of Si/P Exchange. Once the Si atoms have been mobilized as Si(OH)4 extra-framework species, they can insert into the position of framework P atoms. In this insertion the P atom is “kicked out” by the Si atom in a concerted manner. Figure 6 shows a schematic representation of this Si/P exchange. The free energy profile of the Si/P exchange with schematic representations of the intermediates is shown in Figure 7. The zero level of this process is defined to be the sum of the free energy of 19 and Si(OH)4 moving freely in the AlPO-34 structure. The zero-level is labeled 20 in Figure 7. In 21 (12 kJ mol−1) the Si(OH)4 species interacts with the water molecules of 19 through H-bonds. The free energy change from 20 to 21 results from two main contributions: the enthalpic stabilization through H-bonds between the Si(OH)4 species and the water molecules (−41 kJ mol−1) and the entropic destabilization caused by loss of degrees of freedom (DOF) of the Si(OH)4 species (+53 kJ mol−1). 2090


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The Journal of Physical Chemistry C while the barriers associated with proton transfers between the SiO4 and the PO4 moieties are significantly lower. These results make chemical sense. Disrupting the framework of a chabazite structure should require more energy than proton transfers between defects.

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DISCUSSION Mobility of Si and P Framework Atoms in the SAPO Material. In the forward direction, the Si/P exchange process shows that a framework P atom can be replaced with an extraframework Si(OH)4 species. In this process, the framework P atom is converted into an extra framework H3PO4 species which is free to diffuse through the micropores and replace a Si framework atom according to the reverse Si/P exchange process. The Si/P exchange process, therefore, explains how framework Si atoms may move to different framework positions in the SAPO material. This mobility requires, however, that extraframework Si or P species are already present in the micropores. In the rest of this subsection we elaborate on how the extra-framework species may become available in the micropores. In the Results section we have seen that Si(OH)4 species can be mobilized through a desilication and a subsequent H3PO4 insertion. Figure 8 shows a schematic representation of these two processes.

Figure 9. Schematic representation of the processes explaining the mobility of the T atoms in SAPO-34.

However, in the cases where the hydrogarnet defect is generated as a result of the desilication, the number of extraframework species will persist, allowing further Si/P and P/Si exchanges. Si Island Formation. The continued Si/P and P/Si exchanges may explain how the P and Si framework atoms move in SAPO structures. This mobility implies that the isolated Si atoms may cluster together to form Si islands. This Si island formation takes place in two stages. The first stage involves a stepwise construction of a microzeolite (consisting of five T atoms) inside the SAPO material. This process is shown schematically in Figure 10. The second and final stage of the Si island formation takes place through a replacement of the Al atom in the center of the microzeolite with a Si atom. A schematic representation of this Si/Al exchange is shown in Figure 11. The extra-framework Al species liberated in the Si/Al exchange cannot be inserted in the place of a Si or P framework atom. Instead, it may diffuse to the external surface as experimentally observed during dealumination of zeolites.33 In this proposed mechanism of the Si island formation, we have only computed the free energy profile of the first Si/P exchange. For the following three Si/P exchanges, we have assumed mechanisms similar to the first. Figure 7 shows that the equilibrium of the Si/P exchange is shifted toward Si being an extra-framework atom. Formally, the Si/P exchange is the sum of the Si insertion 10 (−94 kJ mol −1 ) and the dephosphoration (163 kJ mol−1). The total free energy change of these two processes (69 kJ mol−1) agrees well with the free

Figure 8. Schematic representation of the desilication and the insertion of H3PO4 into the hydrogarnet defect.

A framework Si atom can therefore be mobilized once a H3PO4 extra-framework species is available. We have argued that H3PO4 species can be generated from P atoms located in defects or at the external surface of the crystal. Later on in the process, the H3PO4 species can be generated in the Si/P exchange. When putting all this information together, a mechanistic picture with certain analogies to a radical reaction emerges. Figure 9 shows that the desilication and the H3PO4 insertion can be thought of as the initiation step. In these two steps, the extra-framework species Si(OH)4 is generated. Furthermore, Figure 9 shows how the Si/P exchange can be thought of as the propagation step. Once an extra-framework species, Si(OH)4 or H3PO4, is available, the Si/P and P/Si exchanges can, in principle, go on indefinitely. In that sense, the extra-framework species show certain analogies with the unpaired electrons in a radical reaction. As long as they are present, the reaction will continue. Alternatively to exchanging with a framework atom, both Si(OH)4 and H3PO4 may enter into a hydrogarnet defect. 2091


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will be different from zero. However, there are reasons to believe that these subsequent forward + reverse Si/P exchanges are close to thermoneutral. Zokaie et al. reported that a SAPO34 structure with Si atoms in a next-nearest-neighbor configuration (Si−O−Al−O−Si, structure 3 in Figure 10) is stabilized by 10 kJ mol−1 relative to a structure with isolated Si atoms.34 On the basis of shell model potential calculations, Sastre et al. reported that a structure represented by structure 4 (Figure 10) is 4 kJ mol−1 higher in energy than structure 2.35 This indicates that the distribution of the structures shown in Figure 10 is approximately uniform. The final stage of island formation is the exchange of the intraisland Al atom for Si (see Figure 11). Previous computational and experimental results indicate that the corresponding process in a zeolite is feasible.36,37 The Si/Al exchange might occur in a concerted manner similar to the Si/P exchange; this possibility is currently unexplored. Alternatively, a two-step process may take place: The Al atom is first removed from the surrounding Si atoms through a hydrolysis by four water molecules. The hydrogarnet defect left after the removal of the Al atom can then be healed by the extra-framework Si(OH)4 species.33,37 Such a process is analogous to the desilication followed by the insertion of a H3PO4 species. Malola et al.36 studied the dealumination/desilication of zeolites computationally. They studied a stepwise hydrolysis of the Al/Si atom by four water molecules. The dealumination is analogous to the removal of the Al atom from the surrounding Si atoms. Furthermore, the reverse desilication is analogous to the insertion of the Si(OH)4 species into the hydrogarnet defect. The computational results by Malola et al. further show that the dealumination is thermodynamically slightly favored over the desilication. When applying this to the Si/Al exchange, we see that the equilibrium should be shifted toward the Si island structure. This fact together with the thermoneutrality of the Si/P exchanges shows that the Si island formation in SAPO-34 is thermodynamically favored. This is in agreement with experimental observations. Influence of the Reaction Temperature. To get a rough idea of how the Si island formation depends on temperature and pressure, we studied the temperature and pressure dependence of the desilication using microkinetic modeling. The desilication process involves four water adsorptions and one Si(OH)4 desorption. Its temperature dependence is determined by two opposing factors. As the temperature increases, the water adsorption equilibria are shifted toward the gas phase. Consequently, at higher temperatures, the free energy span26 of the desilication increases. At the same time, the rate constants of the elementary reactions increase with increasing temperature. At higher temperatures, the desilication is therefore promoted by the kinetics and disfavored by the thermodynamics. Because the desilication involves four water molecule adsorptions, both the rate and the equilibrium of the desilication show a strong dependence of the water vapor pressure. Table 1 below shows the forward rate, r+, and the amount of Si(OH)4 in the micropores, nSi(OH)4, at equilibrium as a function of temperature and pressure. The values of the temperatures and pressures correspond to those used in the modeling above (298 K and 0.02 atm) and to those relevant for the MTO process (623−873 K and 1−6 atm24). From the information in Table 1, we can draw the following conclusions about the pressure and temperature dependence of

Figure 10. Schematic representation of the sequential Si/P exchanges leading to a microzeolite (of five T atoms) within the SAPO structure. The Si and P atoms are colored to clarify how the microzeolite is formed.

Figure 11. Schematic representation of the Si/Al exchange leading to the Si island within the SAPO structure.

energy change of the Si/P exchange (53 kJ mol−1). The discrepancy between the two free energy values is caused by differences in stoichiometry and geometry. The higher endergodicity of the dephosphoration compared to the desilication may be explained by the acid strength of the extra-framework species being formed. H3PO4 (pKa = 2.1) is more acidic than Si(OH)4 (pKa = 9.8) and has therefore a higher tendency to insert into the hydrogarnet defect. Furthermore, because of the thermodynamic relation to the Si/P exchange, the preference for Si being an extra-framework species is also explained by differences in acid strength. The equilibrium position of the Si/P exchange disfavors the substitution of a framework P atom with an Si atom. This has, however, small consequences for the mobility of the Si framework atoms. Once (a tiny amount of) H3PO4 is liberated, it will relatively easily mobilize another framework Si atom according to the reverse Si/P exchange in Figure 7. The net result of the forward and reverse Si/P exchange is that a framework Si atom has moved from one framework position to another one. Because the initial and final states are identical, the free energy change of the forward + reverse Si/P exchange is zero. In the subsequent Si/P exchanges (Figure 10), the initial and final stages are not identical, and their free energy changes 2092


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The desilication involves four water molecule adsorptions. The dephosphoration from a defect/external surface involves adsorption of three water molecules. The Si/P exchange does not involve any water molecule adsorptions, but two water molecules are required to be adsorbed for the reaction to proceed. Because of the strong entropy dependence of the water adsorptions, the desilication should be slower in the temperature range of the MTO process compared to the dephosphoration and the Si/P exchange. Therefore, the temperature and pressure dependence of the Si island formation should to a large degree be determined by the temperature and pressure dependence of the desilication. Suggestions on How To Inhibit Si Island Formation. From the increased mechanistic insight into the Si island formation, we may formulate hypotheses on how to inhibit it. By studying the reaction network in Figure 9, two inhibition strategies emerge: (1) The desilication + H3PO4 insertion, the “initiation step”, can be inhibited. (2) The Si/P exchange can be inhibited. As discussed above, the presence of the H3PO4 species is essential to promote desilication. Furthermore, we have identified internal defects as one likely source of such species. To inhibit the “initiation step”, it would therefore be highly interesting to synthesize a series of SAPO-34 materials where the concentration of defects is varied in a systematic manner and to subject these materials to steam treatments and extensive characterization. However, even though the degree of defectiveness can be controlled in some systems, e.g., by using fluoride as mineralizer,38 this would represent a significant synthesis challenge. It is also unclear how well such a modification would stand up to MTO process conditions in the long term. A second suggestion to inhibit the initiation of the Si island formation is to control the water vapor pressure and the temperature. From the discussion of the microkinetic results, we see that at a water vapor pressure below about 2 atm r+ and nSi(OH)4 decrease with increasing temperature. This should be beneficial because an increase in the reaction temperature will not lead to an increased desilication. Therefore, to inhibit the Si island formation, the water vapor pressure should be kept as low as possible through deliberate process design. Once a H3PO4 or an Si(OH)4 species is available, the Si/P exchange mechanism is determined by the topology (CHA) of the SAPO-34 material. Other topologies may give a different set of relative activation energies and could prove more resistant to Si island formation.

Table 1. Temperature and Pressure Dependence of the Forward Rate, r+, and the Amount of Si(OH)4, nSi(OH)4, at Equilibrium of the Desilicationa temp (K) 298 623 623 623 623 623 623 623 623 623 723 723 723 723 723 723 723 723 723 873 873 873 873 873 873 873 873 873

press. (atm) forward rate at equilib (s−1) 0.02 0.02 0.68 1.00 1.34 2.00 3.00 4.00 5.00 6.00 0.02 0.68 1.00 1.34 2.00 3.00 4.00 5.00 6.00 0.02 0.68 1.00 1.34 2.00 3.00 4.00 5.00 6.00

3.66 3.44 7.05 7.72 8.98 9.24 9.32 4.01 7.57 7.58 1.01 3.61 5.50 8.65 1.23 1.39 4.67 7.24 7.25 3.98 1.68 3.06 6.39 1.76 3.30 6.48 8.02 8.03

× × × × × × × × × × × × × × ×

10−8 10−10 10−8 10−7 10−6 10−4 10−1 102 102 102 10−10 10−8 10−7 10−6 10−3

× × × × × × × ×

102 102 102 10−11 10−8 10−7 10−6 10−3

× 102 × 102 × 102

amount of Si(OH)4 5.14 4.54 9.30 1.02 1.18 1.22 1.23 5.28 9.98 1.00 1.39 4.99 7.60 1.19 1.70 1.92 6.45 9.99 1.00 4.95 2.09 3.82 7.96 2.19 4.11 8.08 1.00 1.00

× × × × × × × × ×

10−12 10−13 10−11 10−9 10−8 10−6 10−3 10−1 10−1

× × × × × × × ×

10−13 10−11 10−10 10−8 10−6 10−3 10−1 10−1

× × × × × × ×

10−14 10−11 10−10 10−9 10−6 10−3 10−1

r+ is given in units of s−1, and nSi(OH)4 is given as a fraction of the total amount of Si species. r+ and nSi(OH)4 are good descriptors of the Si(OH)4 mobility in the SAPO material (see below). The mobility of the Si(OH)4 species further affects the Si/P exchange process which again influences the Si island formation. a

the desilication: For a given temperature in the range 623−873 K, r+ and nSi(OH)4 both increase with increasing water vapor pressure. The increase continues until nSi(OH)4 = 1; that is, all Si in the sample is in the form of Si(OH)4. This behavior is expected based on basic kinetic and thermodynamic principles. The temperature dependence at a given water vapor pressure is slightly more complex. At pressures below 2 atm, r+ and nSi(OH)4 decrease with increasing temperature. At pressures of 2 atm and higher r+ and nSi(OH)4 increase with increasing temperature. From these results, we understand that at low pressures the thermodynamics is mainly governed by the entropy dependence of the water adsorptions. At higher pressures, ΔS in the Gibbs free energy expression becomes more negative. As a consequence, the Gibbs free energy of the water vapor increases, and the adsorption equilibria are shifted toward the adsorbed state. In this situation, the temperature influence on the entropic change of the adsorption is less important. Because of the weaker temperature dependence of the adsorption free energy, r+ and nSi(OH)4 increase with increasing temperature as deduced from basic kinetic and thermodynamic principles.

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CONCLUSIONS We have carried out a computational study on the mechanism of Si island formation in SAPO-34. Three reactions are causing the Si islands to form. The initial step is a desilication through hydrolysis. This desilication generates an extra-framework Si(OH)4 species and a hydrogarnet defect. The hydrogarnet defect is thereafter healed through an insertion of a H3PO4 species. The result of these two initial reactions is a SAPO-34 material with extra framework Si(OH)4 species in the micropores. The Si(OH)4 species may then “kick out” a framework P atom and insert itself into the position of the P atom. This happens through a concerted Si/P exchange. Successive Si/P exchanges explain the mobility of the Si atoms in the SAPO-34 material and also the eventual Si island formation. 2093


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Microkinetic modeling of the desilication suggest that the Si island formation can be inhibited by lowering the water vapor pressure.

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ASSOCIATED CONTENT

S Supporting Information *

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; a detailed description of the reaction paths; input files for the CatMap program. 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. We thank Dr. Rasmus Brogaard for useful discussions about the CatMap program and Dr. Matthias Vandichel for fruitful discussions of the chemistry in general.

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Mechanism of Si Island Formation in SAPO-34 - The Journal of

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