Reaction Mechanisms of the Initial Oligomerization of

Article pubs.acs.org/JPCA

Reaction Mechanisms of the Initial Oligomerization of Aluminophosphate Yan Xiang,† Liang Xin,†,§ Joshua D. Deetz,† and Huai Sun*,†,‡,§ †

School of Chemistry and Chemical Engineering and ‡Ministry of Education Key Laboratory of Scientific and Engineering Computing, Shanghai Jiao Tong University, Shanghai 200240, China § State Key Laboratory of Inorganic Synthesis & Preparative Chemistry, Jilin University, Changchun 130012, China ABSTRACT: The mechanisms of aluminophosphate oligomerization were investigated using density functional theory with the SMD solvation model. Two aluminum species, Al(OH)4− and Al(H2O)63+, and four phosphorus species, H3PO4, H2PO4−, HPO42−, and PO43−, were considered as the monomers for polycondensation reactions. It was found that the most favorable pathway to dimerization was a Lewis acid−base reaction: the aprotic oxygen of phosphoric acid (PO) performs a nucleophilic attack on the central aluminum atom of Al(OH)4−. Using this mechanism as a pattern, plausible dimerization mechanisms were investigated by varying the proticity and hydration of the phosphorus and aluminum monomers, respectively. The relative reaction rates of each mechanism were estimated under different pH conditions. The chain growth of aluminophosphates to trimers, tetramers, and pentamers and the cyclization of a linear tetramer were also investigated. For oligomerization reactions beyond dimer formation, it is found that cluster growth favors the addition of the phosphoric monomers rather than aluminum monomers.

I. INTRODUCTION Microporous aluminophosphates (AlPO4-n) are an important class of zeolitic materials, which were first reported by Wilson and co-workers in 1982.1,2 Since then, more than 200 different structures, including one-dimensional, two-dimensional layered, and three-dimensional open-framework aluminophosphates, have been synthesized under hydro-, solvo-, and iono-thermal conditions.3,4 These materials have potential applications in catalysis, chromatographic separations, and gas adsorption. For example, silicon-doped AlPO4-34 with chabazite topology (SAPO-34) is an important catalyst for the methanol-to-olefins process.5−7 Thus, it is of practical importance to design and synthesize porous aluminophosphate frameworks with tailored architectures and properties, which requires a clear understanding of their formation and crystallization mechanisms. So far, different synthesis mechanisms have been explored using various in situ and ex situ characterization methods, such as X-ray diffraction and scattering, solid-state NMR, atomic force microscopy, and electron microscopy.8−27 According to these studies, oligomerization occurs by rapid condensation reactions between aluminum and phosphor monomers, which form amorphous intermediates in aqueous solution.28 However, because of the limited spatial and temporal resolution of characterization techniques, the mechanisms of aluminophosphate polycondensation are elusive, as well as the mechanisms of aggregation and crystallization. Quantum mechanical (QM) calculations have provided a powerful way to investigate the mechanisms of condensation reactions. Catlow and co-workers studied the formation of siloxane29,30 and aluminosilicate clusters.31−33 Trinh et al. © XXXX American Chemical Society

investigated the reaction mechanism of silane oligomerization.34 However, a theoretical study of the oligomerization mechanisms of aluminophosphates has not been reported in literature. In aqueous solution aluminum exists in a variety of oxohydroxo polyion species Al(OH)n(H2O)m(3−n)+ in which n = 0, 1, 2, 3, or 4, while (n + m) = 4, 5, or 6. On the basis of density functional theory (DFT) calculations it has been shown35 that the primary aluminum species is the hexaaquaaluminum cation (Al(H 2 O) 6 3+ ) at pH < 4.5 and the tetrahydroxyaluminate anion (Al(OH)4−) at pH > 4.5. The mole fraction of phosphate monomers of varying proticity (HnPO4n−3) can be estimated using the experimentally measured dissociation constants of phosphoric acid:36 pK a1 = − log

[H+][H 2PO−4 ] [H 2PO−4 ] = pH − log = 2.12 [H3PO4 ] [H3PO4 ]

(1)

pK a2 = − log

[H+][HPO24−] [HPO24−] = pH − log = 7.21 − [H 2PO4 ] [H 2PO−4 ]

(2)

pK a3 = − log

[H+][PO34−] [PO34−] = pH − log = 12.67 − [HPO4 ] [HPO−4 ]

(3)

Figure 1 is a schematic showing the mole fraction of phosphate monomers as a function of pH, which is consistent with experimental observations.37,38 Received: January 31, 2016 Revised: April 15, 2016

A

DOI: 10.1021/acs.jpca.6b01058 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A

universal solvation model SMD48,49 was employed to evaluate the hydration free energy of solute by ΔGS◦ = ΔG ENP + GCDS

(4)

where GENP refers to the sum of electronic, nuclear, and polarization energy of solute, GCDS represents the total freeenergy change due to cavitation, dispersion forces from the solvent, and estimated structural changes in the local solvent. The free energies were calculated at room temperature (298 K). All computations were performed using the Gaussian09 package.50

III. RESULTS AND DISCUSSIONS Dimerization Reactions. On the basis of Löewenstein’s rule, dimerization reactions between pairs of aluminum or phosphate monomers were not considered. Instead, dimerization reactions between aluminum and phosphate monomers are studied. Four possible reaction mechanisms, sketched in Scheme 1, were considered for the dimerization of Al(OH)4− and H3PO4. Pathways 1/1a represent attacks by the aprotic and protic oxygens of phosphoric acid on the aluminum hydroxide anion. Pathways 1b and 1c describe attacks by the aluminate anion’s oxygens on the phosphorus of H3PO4, which result in a water leaving group either adjacent (1b) or on the opposite side (1c) to location of the attack. Pathway 1 refers to the mechanism in which the aprotic oxygen (PO) in H3PO4 performs a nucleophilic attack on the positively charged central aluminum atom in Al(OH)4− to form an Al−O−P bridge between the monomers, followed by the transferal of a hydroxyl hydrogen of H3PO4 to a hydroxyl group in Al(OH)4−, which subsequently breaks the Al−O bond and forms a leaving water molecule. Pathway 1a is similar, but different in that the hydroxyl oxygen of H3PO4 attacks the aluminum atom in Al(OH)4− to form an Al−O−P bridge bond. Again, a hydrogen atom from a neighboring hydroxyl group is transferred to a hydroxyl group in Al(OH)4−, forming a leaving water molecule. Pathway 1b interchanges the roles of H3PO4 and Al(OH)4−, the oxygen of a hydroxyl group in Al(OH)4− attacks the positively charged phosphorus atom in H3PO4 from the back-side of the PO double bond to form a Al−O−P bridge bond, while a hydroxyl hydrogen in Al(OH)4− is transferred to a hydroxyl group in H3PO4, producing a water molecule. Lastly, pathway 1c denotes the mechanism that the oxygen of hydroxyl group in Al(OH)4− attacks the phosphorus atom in H3PO4 from the side neighboring the PO double bond, forming a pentacoordinated intermediate on the phosphorus atom with a hydroxyl bridge. The proton on the hydroxyl bridge is transferred to form a hydroxyl group on the phosphorus atom via a four-membered cyclic transition state. The intermediate with a pentacoordinated phosphorus atom is unstable as will be explained later, and it rapidly converts to a typical tetracoordinated phosphorus by releasing a water molecule. The free-energy profiles of the four reaction pathways discussed above (1/1a/1b/1c) are illustrated in Figure 2. Pathway 1 occurs in two steps, from a hydrogen-bonded van der Waals (vdW) cluster (1-R) with stabilization energy of −0.9 kcal/mol. In the first step, the transition state (1-TS1) corresponds to the Al−O1 bond formation and H transfer from O2 to O3, leading to an intermediate product (1-IM) in which the generated water still remains on the Al atom. In the second step, the transition state (1-TS2) corresponds to the Al−O3 bond breaking. The initial product is a cluster bonded

Figure 1. Mole fraction of hydrogen phosphate monomers as a function of solution pH. The total mole fraction sum to unity.

In addition to the aluminum and phosphate monomers, other ions in solution (such as H3O+ and OH−) exist, and their concentrations are dependent on pH. To fully understand the mechanisms of aluminophosphate oligomerization we must consider the role of the solvent and ions explicitly. Ab-initio molecular dynamics (AIMD) or reactive empirical force fields (such as ReaxFF37) appear to be required to accurately model oligomerization. However, AIMD is very expensive for simulating chemical reactions in solution, and there is currently a lack of relevant parameters for empirical models. In this work we focus on simple oligomerization reactions of aluminum and phosphate monomers using a continuum hydration model to represent aqueous solution. By considering tetrahydroxyaluminate (Al(OH)4−) and phosphoric acid (H3PO4) as monomers, we first identify the most favorable reaction mechanism of dimerization, which is subsequently used as the pattern to study the dimerization of other pairs of monomers. Furthermore, we estimate the relative rates of different dimerization reactions under different pH. The mechanisms of oligomer (trimer, tetramer, pentamer) growth are also explored. This work provides a microscopic insight into the initial stages of polycondensation. In addition, the data here will provide a foundation for the development of a reactive force field for molecular dynamics simulations to enable simulating polycondensation for larger length/time scales.39−41

II. METHOD AND MODELS The aluminum monomers considered in this work are the hexaaquaaluminum cation (Al(H2O)63+) and the tetrahydroxyaluminate anion (Al(OH)4−). The phosphorus-containing species considered are phosphoric acid (H3PO4) and phosphate anions of varying proticity (H2PO4−, HPO42−, and PO43−). All calculations were conducted using DFT42,43 with Becke’s three parameter exchange functional44 and Lee−Yang−Parr nonlocal correlation functional (B3LYP).45 The Pople basis set 6-311+ +G(d,p) was used to model the molecular electron orbitals. The geometry optimization of all monomers listed above was conducted using the energy-represented direct inversion in iterative subspace algorithm (GEDIIS).46 The obtained structures were confirmed to be energy minima by verifying that only positive eigenvalues exist for the Hessian matrix. Transition-state structures were verified to have only one negative eigenvalue. Intrinsic reaction coordinate (IRC) analysis47 was performed to confirm that the transition state was connected by a reaction pathway to the local minima corresponding to the reactant and product complexes. The B


Article

The Journal of Physical Chemistry A Scheme 1. Pathways to Dimerization of Al(OH)4− and H3PO4

(1A-TS1) and the second transition state corresponds to the dehydration of the aluminum complex (1A-TS2). The overall energy barrier of this pathway is 13.0 kcal/mol. Pathway 1b is a one-step reaction, but is energetically much less favorable than Pathways 1 or 1a with energy barrier of 51.5 kcal/mol. Pathway

with water (1-P). At infinite separation in solution (1-P-inf) the overall reaction energy is −15.3 kcal/mol. The overall energy barrier of this pathway is 9.1 kcal/mol, which is the lowest among the four mechanisms investigated. Pathway 1a also occurs in two steps, in which the Al−O−P bridge is first formed C


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water as a leaving group. The overall energy barrier is 47.7 kcal/ mol. The fact that Pathways 1 and 1a are more favorable than the 1b/1c is consistent with our chemical intuition that phosphorus is a weaker Lewis acid than aluminum. The central atom of the phosphorus monomer is more electronegative than that of the aluminum monomer. Thus, it is less energetically favorable for an oxygen atom to attack a phosphorus atom than an aluminum atom, due to its higher electron density at the reaction site. Since the overall energy barrier of Pathway 1 is lower than those of Pathways 1a−c, the general pattern of Pathway 1 was used to investigate additional reactions later in this study. Additional dimerization reaction mechanisms in which the aluminum atom of the Al(OH)4− monomer is attacked by the aprotic oxygen from H2PO4−, HPO42−, and PO43− are denoted as Pathways 2, 3, and 4, respectively. The reaction mechanisms are similar to the anionic mechanisms of silicate dimerization presented elsewhere.34 Scheme 2 illustrates a dimerization mechanism using H2PO4− as an example. In these reactions, an aprotic oxygen of H2PO4− attacks the aluminum center of Al(OH)4−, forming a pentacoordinate intermediate. There are two transition states; the first one refers to the formation of a P−O−Al bond, and the second one corresponds to the exclusion of a hydroxide ion. The free-energy profiles for Pathways 2−4 are shown in Figure 3. In Pathway 2, the H2PO4− approaches Al(OH)4− to a minimal distance to form a structure stabilized by three hydrogen bonds (2R). In the first step, the transition state (2TS1) corresponds to Al−O1 bond formation, and an intermediate product (2IM) is formed with a pentacoordinate aluminum. The free-energy barrier of the first step is 16.4 kcal/ mol. The second step corresponds to the Al−O2 bond breaking and the hydroxyl group leaving. The free-energy barrier of this step is 15.6 kcal/mol. The products separated at infinite distance (2P-inf1) exist at a high energy level. However, a hydrogen transfer from the dimer to the hydroxyl ion leads to a much more stable set of products (2P-inf2). Unlike Pathway-1, the hydrogen does not transfer simultaneously with the formation of the P−O−Al bond due to the lower acidity of H2PO4−, and the overall free-energy barrier is 28.1 kcal/mol. Pathways 3 and 4 are similar to Pathway 2. The difference arises from larger negative charges on the HPO42− or PO43− ions, which prevent the formation of a stable vdW cluster structure with the negatively charged Al(OH)4− due to strong electrostatic repulsion. Nevertheless, these differences do not appear to increase the overall free-energy barriers, which are 26.5 kcal/ mol for Pathway 3 and 25.8 kcal/mol for Pathway 4. Reactions in which the aluminum atom of Al(H2O)63+ is attacked by an aprotic oxygen from H3PO4 and H2PO4−, denoted as Pathways 5 and 6, were considered. The combinations of the most oppositely charged species Al(H2O)63+ and HPO42−, PO43− were found to be unstable to form vdW clusters using the optimization protocol presented in this work. Because Al(H2O)63+ exists in low pH solution, where HPO42− and PO43− would be largely reduced to H2PO4− or H3PO4, we did not calculate the dimerizations between highly charged monomers. The mechanism of Pathway 5 is sketched in Scheme 3. This pathway is a typical SN2 backside attack reaction, where the Al− O−P bridge is formed and an Al−H2O bond is broken synchronously. The free-energy diagram of Pathway 5 is shown in Figure 4. H3PO4 prefers to stay separated from Al(H2O)63+ rather than form a vdW cluster (5R), whose free energy is 5.5

Figure 2. Gibbs free-energy profiles along (a) pathway 1, (b) pathway 1a, (c) pathway 1b, and (d) pathway 1c of dimerization. Atomic color representations: orange for phosphorus, pink for aluminum, red for oxygen, and white for hydrogen.

1c is more complex as it has three transition states. The first (1C-TS1) represents an attack on the phosphorus atom by a hydroxyl oxygen, the second (1C-TS2) is an intramolecular hydrogen transfer, and the third (1C-TS3) is the formation of D


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The Journal of Physical Chemistry A Scheme 2. Mechanism of Dimerization of Al(OH)4− and H2PO4−

center corresponding to the Al−O1 bond formation and Al− O2 bond breaking simultaneously (5TS). As the Al−O2 bond breaks completely, a water molecule is removed, and a hexacoordinated dimer is formed (5P). This complex is further stabilized by dissociation of the dimer from the water molecule (5P-inf). The overall free-energy barrier of this pathway is 29.0 kcal/mol. The mechanism of Pathway 6 is sketched in Scheme 4. It is similar to an SN1 reaction, where the leaving group’s bond (Al− H2O) breaks and the substitution group’s bond (Al−O) forms in two steps. The free-energy diagram of Pathway 6 is shown in Figure 5. In the first step, the H2PO4− approaches Al(H2O)63+ to form a stable structure due to vdW interaction, where a hydrogen of Al(H2O)63+ transfers to H2PO4− spontaneously (6R). Note that 6R represents H3PO4 and Al(H2O)5(OH)2+ rather than H2PO4− and Al(H2O)63+, which is more favorable with a 0.2 kcal/mol reduction in free energy compared with reference state (6R-inf). The first transition state (6TS1) corresponds to the removal of a water molecule, transforming the aluminum from a six-coordinated into a five-coordinated configuration (6IM). In the second step, the transition state (6TS2) corresponds the P−O−Al bridge bond formation, reversely transforming the aluminum back to a six-coordinated state (6P). The complex is further stabilized by the dissociation of a water molecule from the dimer (6P-inf). The overall freeenergy barrier of this pathway is 10.9 kcal/mol. Table 1 summarizes the free-energy barriers and reaction free-energy changes for the six pathways characterized. Dependence of the Rate of Dimerization on pH. Using the free-energy barriers in Table 1 and estimating the mole fractions of the different monomers at different pH (see ref 40 and eqs 1−3), we may derive the relative initial reaction rate for Pathways 1−6 as a function of pH. The reaction rate constant k(T) for each pathway can be calculated using the Eyring equation:51 k(T ) =

kBT o K exp( −ΔGa /RT ) h

(5)

where kB and h are the Boltzmann and Planck constants, R is the gas constant, and ΔGa is the activation free energy. With the rate constants and mole fraction of each species determined, the initial reaction rate of the six pathways are approximated by power law kinetics

Figure 3. Gibbs free-energy profile along the reaction pathways 2, 3, and 4 of dimerization. Atomic colors are the same as in Figure 2.

ν = k(T )[Al][P]

kcal/mol higher than reference state (5R-inf), although three hydrogen bonds are found in the cluster. The transition state is a septacoordinated (coordinated to seven neighbors) aluminum

(6)

where [P] and [Al] represent the mole fractions of the phosphate and aluminum monomers. The mole fraction of tetrahydroxyaluminate was taken to be unity above a pH of 4.5, E


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The Journal of Physical Chemistry A Scheme 3. Mechanism of Dimerization of Al(H2O)63+ and H3PO4

Figure 5. Gibbs free-energy profile along reaction pathway 6 of dimerization (B3LYP/6-311++G(d,p)-SMD). ΔG is the free-energy change in aqueous solution relative to 6R-inf. Atomic colors are the same as in Figure 2.

Figure 4. Gibbs free-energy profile along reaction pathway 5 of dimerization (B3LYP/6-311++G(d,p)-SMD). ΔG is the free-energy change in aqueous solution relative to 5R-inf. Atomic colors are the same as in Figure 2.

Table 1. Summary of Overall Free-Energy Barriers (in kcal/ mol) and Free-Energy Differences between Reactants and Products of All Possible Dimerization Pathways

whereas hexaaquaaluminum was taken to be unity below this value. Using the reaction rates for all pathways, the relative rate is defined as r(Pathway‐i) =

ν(Pathway‐i) ∑j ν(Pathway‐j)

pathway 1 2 3 4 5 6

(7)

The relative reaction rates versus pH values are shown in Figure 6. In acidic conditions, Pathway 6 is the most favorable. Pathway 1 dominates in neutral and weak basic conditions (6 < pH < 10), and Pathways 3 and 4 become the major reactions in strong basic conditions. Oligomer Growth. Oligomerization beyond dimers including the formation of trimers, tetramers, pentamers, and cyclic tetramers was investigated. The energetics of the oligomerization reactions are calculated using the same pattern as Pathway 1: the aprotic oxygen of phosphoric acid or aluminophosphate oligomers attacks the aluminum atom of monomers. A detailed

description Al(OH)4− Al(OH)4− Al(OH)4− Al(OH)4−

+ + + +

Al(H2O)63+ Al(H2O)63+

H3PO4 H2PO4− HPO42− PO43− + H3PO4 + H2PO4−

overall free-energy barrier

ΔG

9.1 28.1 26.5 25.8 29.0 10.9

−15.3 −5.3 3.4 7.7 −3.5 −8.0

description of these mechanisms is omitted because all the reactions occur by the same mechanism as Pathway 1. The free-energy reaction barriers and changes of all oligomerization reactions are listed in Table 2. The reactions are classified into two categories: cluster growth by addition of either a phosphorus (phosphoric acid) or aluminum (tetrahydroxyaluminate) monomer. Although all of these reactions

Scheme 4. Mechanism of Dimerization of Al(H2O)63+ and H2PO4−

F


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Figure 7. Transition states of the reaction (a) Al−P + P → P−Al−P. and (b) Al−P + Al → Al−P−Al. Hydrogen bond was plotted in dashed lines, and the distance was denoted in the figure. Atomic colors are the same as in Figure 1.

Annulation of the linear structure to a cyclic tetramer is moderately favorable with a free-energy barrier of 10.8 kcal/mol and heat of reaction of −13.1 kcal/mol.

Figure 6. Relative initial reaction rates of six dimerization reactions with different pH.

follow a similar mechanism, a clear difference between growth by phosphorus or aluminum is obtained. The activation energy barriers of phosphor addition (6.0 to 8.8 kcal/mol) are generally lower than those of aluminum addition (10.5 to 19.4 kcal/mol). In addition, the reactions are more exothermic for phosphor addition (−10.4 to −12.0 kcal/mol) than for aluminum addition (−1.3 to −8.6 kcal/mol). Therefore, oligomer growth by the addition of phosphor monomers is more favorable than aluminum from the viewpoint of both kinetics and thermodynamics. The difference in reaction energy reflects the fact that moieties of P−Al−P are more stable than Al−P−Al, which is consistent with the fact that synthesized aluminophosphates are likely to have an Al/P ratio less than one.4 The difference between the free-energy barriers for the two kinds of additions can be understood by comparing the structures of the transition states from dimer to trimer as shown in Figure 7. It is important to note that the dimer (Al−P) forms a six-member ring via internal hydrogen bond and a coordination bond between an aprotic oxygen and aluminum as shown on the left side of the complex in Figure 7a. In the phosphor addition reaction, the phosphor monomer can be easily rotated so that the transition state is stabilized by two hydrogen bonds between the two reactants (forming an eight-member ring). However, in the aluminum addition reaction (Figure 7b), the rotation of the phosphor group in the aluminophosphates is restricted, with only one hydrogen bond formed with the aluminum monomer. Finally we calculated the energetics of the cyclization of a linear tetramer, as the cyclic tetramer is one of the most important sub-building units found in aluminophosphate zeolites.52,53 The mechanism is similar to Pathway 1, and the free-energy diagram of the reaction is shown in Figure 8.

Figure 8. Gibbs free-energy profile along the tetramer cyclization reaction pathway (B3LYP/6-311++G(d,p)-SMD). ΔG is the Gibbs free energy in aqueous solution relative to 7R. Atomic colors are the same as in Figure 2.

IV. CONCLUSIONS Reaction mechanisms related to the initial stages of aluminophosphate oligomerization were investigated by means of DFT/SMD calculations. The nucleophilic attack by the aprotic oxygen of phosphoric acid on the central aluminum atom in tetrahydroxyaluminate is found to be the most favorable mechanism, because aluminum is more electropositive than phosphorus and serves as a better Lewis acid to accept lone-pair electrons from oxygen. On the basis of this mechanism, six pathways of dimerization between different aluminum and phosphorus monomers were evaluated. Oligomerization beyond dimers shows a pattern that the

Table 2. Calculated Activation Barriers (in kcal/mol) and Free-Energy Differences for Condensation Reactions Forming Clusters from Dimers to Pentamers condensation reaction Al−P Al−PAl Al−PAl−P P−AlP Al−P P−AlP Al−PAl−P Al−PAl

+ + + + + + + +

P P P P Al Al Al Al

→ → → → → → → →

P−AlP Al−PAl−P P−AlP−AlP Al(P)3 Al−PAl Al−PAl−P Al−PAl−PAl P(Al)3 G

overall barrier

ΔG

7.7 7.6 6.0 8.8 12.7 19.4 10.5 12.5

−12.0 −11.0 −10.4 −11.5 −7.4 −1.3 −8.6 −6.8 DOI: 10.1021/acs.jpca.6b01058 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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addition of phosphor monomer to a cluster is more energetically favorable than the addition of aluminum monomer from the perspective of both thermodynamics and kinetics. This difference is because the “free” phosphorus monomer can form a more stable transition state than a condensed and restricted phosphorus, and the P−Al−P moieties are thermodynamically more stable than Al−P−Al moieties. For tetramers, the four-member ring closure reaction is found to be modestly favorable. The relative reaction rates of different pathways to dimerization under different pH was estimated using the relative concentrations of monomer and rate constants derived from Eyring equation. This qualitative analysis of reaction mechanisms under different pH conditions is an approximation because the solvent, and other ions in the reactive solution were represented by the continuum solvation model or ignored. For example, our calculation indicates that hydrogen bonding is indispensable to the oligomerization reactions, and the inclusion of real water molecules in the reactive complex may cause reaction energy barriers to change from what is shown here. Nevertheless, the computational data obtained in this work has established a cornerstone for understanding the mechanism of aluminophosphate oligomerization at early stages and provides basic data for developing reactive force fields, which can be used with explicit solvent to further explore the mechanisms of aluminophosphate synthesis.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-21-5474-8987. Fax: +86-21-5474-1297. E-mail address: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge grant support through National Natural Science Foundation of China (Nos. 21073119, 21173146, and 21473112), National Basic Research Program of China (973 Program; No. 2014CB239702), and computational resources from Center for High Performance Computing at Shanghai Jiao Tong University.

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REFERENCES

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Reaction Mechanisms of the Initial Oligomerization of

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