Article pubs.acs.org/JPCC
Stabilization of Silicon Islands in Silicoaluminophosphates by Proton Redistribution Mahsa Zokaie,† Unni Olsbye,† Karl Petter Lillerud,*,† and Ole Swang*,†,‡ †
inGAP Centre for Research-Based Innovation, Department of Chemistry, University of Oslo, P.O. Box 1033, Blindern, N-0315 Oslo, Norway ‡ Department of Process Chemistry, SINTEF Materials and Chemistry, P.O. Box 124 Blindern, N-0314 Oslo, Norway ABSTRACT: The relative stabilities of different proton distributions around a five-atom silicon island in silicoaluminophosphate-34 (SAPO-34) have been investigated by periodic molecular mechanics (MM) energy minimization calculations, and the MM calculations were validated using density functional theory (DFT). SAPO-34 has chabazite topology with only one symmetrically independent tetrahedral site (T-site) and four unique oxygen sites. The preferred position of the proton at isolated acid sites has been the subject of both experimental and computational studies. In previous computational studies of silicon islands, it was assumed that the protons, necessary to keep the silicon island neutral, would occupy the same positions as those preferred for solitary silicon atoms. We have studied all 108 possible proton distributions around a five-atom silicon island in SAPO-34. The results indicate that the proton placement is critical for stability, as the limit deviation in our data set is as high as 90 kJ/mol. Careful analysis of the different structures afforded criteria for stability of the proton configuration around a five-atom silicon island in SAPO-34. Preliminary calculations indicate that these findings are transferable to other topologies and larger islands.
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INTRODUCTION Crystalline nanoporous framework materials (CNFMs) have attracted much attention from both academic and industrial materials science over the past half century. It is the molecular dimension of the pores within these materials, combined with the possibility of functionalizing their huge internal surface, that continues to hold both fascination for academics and promise for industrialists. When these properties are properly married, such as in the most important subgroups of CNFMs, zeolites and microporous aluminophosphates (AlPOs), these materials have found utilization in areas as diverse as water softening, environmental cleanup, catalytic cracking, and the production of value-added products such as olefins and gasoline from fossil feedstock. Active sites are typically formed by substitution of tetrahedral atoms (T-atoms) in the lattice. For silicoaluminophosphate (SAPO) materials, which are the subject of this study, active sites appear in the material when some phosphorus atoms of the aluminophosphate framework are replaced by silicon− proton pairs. This is similar to aluminum substitution for silicon in zeolites, but with an important difference: In zeolites, aluminum−aluminum nearest neighbors are unstable and are not observed (Löwenstein’s rule). No such limitations are placed on silicon−silicon nearest neighbors in SAPOs, however, and their existence has been thoroughly demonstrated.1−3 Silicon−silicon neighbors will create a nanosized zeolitic domain within the SAPO material, and such domains are called “silicon islands”. Their formation is thermodynamically © 2012 American Chemical Society
allowed, as demonstrated by the fact that SAPOs will phaseseparate into AlPO and zeolite phases upon prolonged heating.1,2 An interesting point is that the formation of silicon domains will alter the acidic properties of a SAPO material. An isolated silicon defect in a SAPO creates a Brønsted acid site that is slightly less acidic than the analogous site in a zeolitic material with the same topology and acid density.4,5 This gives a material with small but important differences in catalytic activity, selectivity, and deactivation properties. The acidity due to isolated silicon atoms is lost when they merge, resulting in permanent deactivation, but new acidic sites will appear at the silicon-island−AlPO interface.6 It has been suggested that these “island sites” have a significantly greater acidity compared to the acid sites arising from single silicon atom substitutions.7,8 Protons migrate rather easily in zeotype materials.9,10 Calculations have shown the possibility of proton jumps between neighboring oxygen atoms of the same tetrahedron (local jump) and translational proton motion between nonneighboring oxygen atoms, which happens even at room temperature.11−14 In the case of proton jumps between oxygen atoms linked to the same T-site, barriers on the order of 60 kJ/ mol have been found for a system in the absence of water.15,16 Under hydrothermal conditions (several hours at roughly 400 K), this barrier may be overcome thermally. Additionally, Received: November 2, 2011 Revised: February 18, 2012 Published: March 22, 2012 7255
dx.doi.org/10.1021/jp210537v | J. Phys. Chem. C 2012, 116, 7255−7259
The Journal of Physical Chemistry C
Article
Density Functional Theory (DFT) Calculations. The Dmol3 program32 was used for periodic DFT calculations. Single-point energy calculations were carried out using the PW91 density functional33,34 in conjunction with double numeric plus polarization (DNP) basis sets. Crystal Models and Silicon Islands. A silicon island formed by replacing four Al and one P atom with Si was chosen as the silicon island model (Figure 2). This is believed to be the
proton mobility may be catalyzed by water. During the subsequent calcination, even higher thermal energies are available, and we conclude that it may be safely assumed that the protons surrounding a silicon island will attain the thermodynamically most favored positions. In earlier work on the subject of silicon island stability,17−19 it was assumed that the protons, necessary to keep the silicon island neutral, would occupy the same positions as those preferred for solitary silicon atoms. Preliminary computational studies led us to doubt that assumption. In the present work, we have investigated the relative stabilities of different proton distributions around a fiveatom silicon island in SAPO-34. SAPO-34 has chabazite topology with only one symmetrically independent tetrahedral site (T-site) and four unique oxygen sites (Figure 1). The protons introduced to ensure
Figure 2. A schematic representation of the selected five-atom silicon island. This island has a uniquely defined topology in the chabazite structure.
smallest stable silicon island in SAPOs.17 The hexagonal representation of SAPO-34 with cell parameters a = 13.68 Å, b = 13.73 Å, and c = 14.83 Å, with a stoichiometry of Si5Al17P14O72H3, was used in the lattice energy calculations. Three protons are needed to provide charge neutrality. The central Si atom is connected to four other Si atoms through oxygen bridges; this silicon is analogous to silicon in a siliceous zeolite, and no protons will bond to its associated oxygen atoms. The four remaining silicon atoms have three oxygen atoms each, to which protons can bond. If we assume that one tetrahedron will accommodate only one proton, we arrive at 108 different possible structures, only differing in the proton positions. Each of these structures was optimized using molecular mechanics.
Figure 1. SAPO-34 structure with four different crystallographic oxygen atoms. Oxygen atoms may assume the following positions: O(1): two four-membered rings (4-MRs) and one 8-MR, O(2): one 4MR, 6-MR, and 8-MR, O(3): two 4-MRs and one 8-MR, and O(4): one 4-MR and two 8-MRs. The labeling scheme is taken from Ito et al.28
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charge neutrality after silicon substitution may coordinate to one of the four different oxygen atoms. The preferred position of the proton at isolated acid sites has been the subject of both experimental20−24 and computational4,19,25−27 studies. The relative energies of the different positions are, in all estimates, less than 10 kJ/mol, indicating that they cannot explain the large energy variations between different proton configurations around silicon islands (vide inf ra). The stability of the silicon island is dependent on the proton distribution to a surprising extent. We arrive at a number of criteria that a proton distribution must satisfy in order to be energetically close to the ground state.
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RESULTS AND DISCUSSION The calculated relative energies for the 108 structures are plotted in Figure 3 (the most stable structure has been chosen as a zero energy reference). The results show that the proton
COMPUTATIONAL DETAILS
Molecular Mechanics Calculations. Periodic molecular mechanics geometry optimizations have been carried out at constant pressure using the General Utility Lattice Package (GULP)29 as implemented in the Materials Studio 5.0 program system.30 The shell model potential of Catlow31 was employed for all calculations.
Figure 3. Relative energy of different structures calculated by molecular mechanics. Black points are structures belonging to at least one of the exclusion categories, and red points are the remaining stable structures. 7256
dx.doi.org/10.1021/jp210537v | J. Phys. Chem. C 2012, 116, 7255−7259
The Journal of Physical Chemistry C
Article
placement is critical for stability. To our surprise, the difference between the most stable and least stable configuration is as high as 90 kJ/mol. An attempt to make a simple correlation between stability and electrostatic repulsion between the three protons based on their mutual positions failed. Careful analysis of the different structures yielded the following criteria for excluding structures as unstable (see Figure 4): a. Two protons connected to oxygen atoms in the same ring, disregarding ring size (4-ring, 6-ring, and 8-ring).
Figure 5. Comparison of DFT and MM results for representative structures.
and the standard deviation (σ) is 6.4 kJ/mol). The DFT calculations reproduce the qualitative picture of a large energy difference between the different proton placements. As mentioned in the Introduction, previous work on silicon islands has been carried out on models in which the protons are connected to the same oxygen as those preferred for solitary silicon atoms,17−19 as seen in the work by Sastre et al.3 The latter conclude that the formation of five-membered silicon islands is roughly thermoneutral. In Figure 6, their structure
Figure 4. Examples of unstable structures and their range of relative energies: (a) protons connected to oxygen atoms in the same ring, (b) protons connected to tetrahedra located in the same double six ring, (c) proton pointing to open space (connected to O(4)), and (d) proton connected to O(2).
Figure 6. Five-atom silicon island in SAPO-34 with two different proton distributions. (A) Taken from literature.3 (B) The most stable structure of this study.
b. All three protons connected to oxygen atoms connected to silicon atoms in the same double 6-ring. c. At least one proton pointing toward large zeolite cavities or rings of larger than eight T-atoms (structures with proton on O(4), which is the common oxygen of one 4ring and two 8-rings). d. At least one proton connected to an oxygen atom in the O(2) positions (O(2) is the oxygen that is shared by one 4-ring, one 6-ring, and one 8-ring.) Excluding all these energetically unstable structures, the remaining structures (red points in Figure 3) were all within 20 kJ/mol of the ground state. It should be mentioned that our criteria (specifically, criterion d above) excluded four stable (
