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2009, 113, 2647–2652 Published on Web 01/23/2009
Role of Water in Silica Oligomerization Thuat T. Trinh,† Antonius P. J. Jansen,† Rutger A. van Santen,† and Evert Jan Meijer*,‡ Schuit Institute of Catalysis, Laboratory of Inorganic Chemistry and Catalysis, EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands, and Van’t Hoff Institute for Molecular Sciences and Amsterdam Center for Multiscale Modeling, UniVersity of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands
J. Phys. Chem. C 2009.113:2647-2652. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 01/29/19. For personal use only.
ReceiVed: August 8, 2007; ReVised Manuscript ReceiVed: January 6, 2009
The silicate oligomerization reaction is key to sol-gel chemistry and zeolite synthesis. Numerous experimental and theoretical studies have been devoted to investigating the physical chemistry of silicate oligomers in the prenucleation stage of siliceous zeolite formation. Most of the reported computational studies of silica oligomerization employ a quantum chemical phase model supplemented with a continuum model for the solvent. We report a density-functional theory based molecular dynamics simulation of silica oligomerization in a bulk solution of explicit water molecules. Our study provides thermodynamics, kinetics, and mechanism of the reaction pathway for the anionic bond formation of siliceous oligomers. We reveal that in the water cleavage step a direct and a water mediated proton transfer pathways may occur and show that changing from a continuum to a explicit water model the rates of SiO-Si bond formation of linear and three-ring oligomers are significantly enhanced, whereas the overall thermodynamics becomes less favorable. Introduction The silica condensation reaction is the key elementary reaction step of silica sol-gel chemistry1,2 as well as alumino-silicate synthesis.3 Understanding how alumino-silicates as zeolites nucleate and grow is of fundamental scientific and technological importance.Numerousexperimental4–11 andtheoreticalstudies12–20,32,33 have been devoted to investigate molecular aspects of the kinetics and thermodynamics of silicate oligomers at prenucleation conditions. There is still limited information on the activation barriers that control the rate of the SiO-Si formation step. Previous gas-phase computational studies of the reaction showed that the formation of inter- or intramolecular hydrogen bonds is an important aspect. From gas phase/continuum calculations,20 we found that especially the relative rates of formation of silicate ring oligomers and linear oligomers were affected by differences in the intramolecular hydrogen bonding. These calculations (from here on referred to as continuum solvent model) indicated significantly higher barriers for ring formation than for linear oligomer formation. This can not be readily reconciled with experimental observations in aqueous solution. Computational studies modeling the aqueous environment by explicit water molecules are essential for a better understanding of the role of such hydrogen bonding effects.21 Here we report on an ab initio * To whom correspondence should be addressed. Tel.: (+31)-20525.5265. Fax: (+31)-20-525.5604. E-mail:
[email protected]. † Eindhoven University of Technology. ‡ University of Amsterdam.
10.1021/jp076372c CCC: $40.75
density-functional theory based molecular dynamics (DFT-MD) simulation study of the silica condensation reaction in aqueous solution. Computational Details Earlier we concluded, in agreement with experiment, that silica oligomerization proceeds through an anionic intermediate species.20 Therefore, we used this anionic pathway (Scheme 1) to study the formation of dimer, linear trimer, and 3-ring silicate oligomers. This is a two step pathway with first a SiO-O bond formation followed by the detachment of a water molecule. Simulations were performed employing the Car-Parrinello22 method as implemented in the CPMD package.23,24 The electronic structure was calculated using DFT and employing the BLYP functional.25,26 The BLYP functional has proven to give an accurate description of the structure and dynamics of water27–29 and of the silica-water interaction.30 We simulated a system consisting of silicic acid Si(OH)4, its deprotonated form Si(OH)3O-, and 64 water molecules in a periodically replicated cubic cell. The simulation cell size was fixed, yielding the experimental density of the solution at ambient condition. In this setup, the first solvation shell and part of the second solvation shell of the silica dimer and trimer are present, ensuring a reasonable accurate description of the aqueous solvation of the oligomers. The negative charge is countered by a uniform positive background charge as to yield a neutral system. The temperature is set at T ) 350 K and imposed with a Nose´-Hoover thermostat. Simulation trajectories were typi 2009 American Chemical Society
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SCHEME 1: Intermediates According to the Anionic Mechanism of the Silica Condensation Reaction (Schematics)a
a
Two proton transfer mechanisms are compared.
TABLE 1: Calculated Free Energy Barriers (kJ/mol) for the Condensation Reactions Forming Silicate Species of Dimer, Linear Trimer, and 3-Ring via the Anionic Mechanismc
a Reference 16; calculated single-point energies values using B3LYP/6-31+G(d,p), COSMO model for solvation energy. The difference in relative reaction energy calculated (single-point energy calculation with the same silica geometry in gas phase) between CPMD/BLYP and B3LYP was estimated around 5 kJ/mol. b The thermodynamic property of the reaction is not evaluated from the initial reactant and the final product but from the stable intermediate state in reaction process. c Eact1 is the activation barrier of SiO-Si bond formation step, and Eact2 is the activation barrier of water removal step. The overall-barrier is the difference in energy between highest point and the lowest point along the reaction coordinates.
cally 15 ps with a prior equilibration of 1.5 ps allowing for sufficient orientational and translational relaxation of the water molecules. As the rate of condensation reaction is outside the time scale accessible to ab initio molecular dynamics, the reactive events were enforced using the method of constraints. Starting from an equilibrated configuration, the formation of a five-coordinated intermediate was controlled by varying the distance between the O1 and Si2 atoms stepwise (see Scheme 1, showing the dimer formation). Subsequently, the reactive event of water
removal was controlled by a stepwise variation of the Si2-O3 distance. The free energy profile along the imposed reaction pathway is obtained by integration of the constraint force.31 Results and Discussion Table 1 lists the calculated free energy data of the formation of the dimer, linear trimer, and ring trimer. Figure 1 shows representative snapshots for the second step of the reaction with a water detaching from the intermediate silica oligomer. Figure 2 shows the calculated force profile and associated free-energy
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Figure 1. Snapshots of representative configurations of ab initio molecular dynamics simulations of second step (water removal) anionic mechanism to form the dimer (a), linear trimer (b), and ring trimer (c). More detailed snapshot are provided in the Supporting Information. White, red, and yellow indicate hydrogen, oxygen, and silicon atoms, respectively. Numbers indicate bond lengths [Å]. The symbols d1 and d2 indicate the bonds mentioned in Figures 2-4. a1, b1, c1: The intermediate 5-fold silicon complex. This species is stable in aqueous media. a2, b2, c2: During the process: differences in hydrogen bonds network and interaction between silicate and water molecules. a3, b3, c3: Water cleavage has been completed. The geometry is full relaxed after 30 ps.
profile for the dimer formation, with the first point of enforced pathway taken as reference. The first step (SiO-Si bond formation) and the second step (cleavage of Si-OH) have a similar activation barrier of ∼45 kJ/mol. Analysis of the dimerization process indicates that the water molecules stabilize the silicate species by strong hydrogen bonds. The hydrogen bond network of surrounding water molecules assists the water removal process that concludes the SiO-Si formation. In this step, the leaving hydroxyl group combines with an internal proton of the dimer to form a water molecule (Figure 1 and internal proton transfer in Scheme 1). The contrast with the continuum solvent model is significant: incorporating explicit
water molecules lowers the barriers by 10-20 kJ/mol with a more enhanced reduction of the second barrier. The latter is due to the stronger stabilization of the leaving hydroxyl group by hydrogen bonds by explicit water molecules. Interestingly, the presence of water molecules not only has an impact on the reaction barrier but also affects the overall thermodynamics of SiO-Si formation with the reaction becoming less favorable in the explicit water model compared to continuum solvent model. The calculated free energy profile of the consecutive oligomerization step from dimer to linear trimer is shown in Figure 3. The activation barrier for the first step, i.e. the SiO-Si bond
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Figure 2. Calculated constraint force and associated free-energy profile along the reaction path of silica dimerization. The free-energy profile is obtained by integrating the connecting line through the calculated constraint force points.
Figure 3. Calculated constraint force and associated free-energy profile along the reaction path of the formation of the linear trimer. The freeenergy profile is obtained by integrating the connecting line through the calculated constraint force points.
formation, is similar to that in the dimerization reaction, wheras the barrier for the second step in which a water molecule detaches is ∼10 kJ/mol lower. This is due to an essential difference in the reaction mechanism, where in the explicit water model the leaving hydroxyl group combines with a proton from a hydrogen bonded solvent water that in turn receives a proton from the trimer oxygen: the proton transfer is mediated by a solvent water molecule12 (Figure 1 and external proton transfer
in Scheme 1). Note that this effect could not be captured by the continuum water model that yields consequently similar barriers for the water removal step in the dimer and trimer formation. Figure 4 shows the calculated free energy profile of the internal condensation reaction from the linear to the ring trimer. The barrier for the SiO--Si bond formation in the ring closure reaction is ∼10 kJ/mol lower than in the oligomerization
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Figure 4. Calculated constraint force and associated free-energy profile along the reaction path of the formation of the silica ring trimer. The free-energy profile is obtained by integrating the connecting line through the calculated constraint force points.
process. The mechanism of the water removal step is similar as in the dimerization reaction, proceeding via an internal proton transfer, though with a slightly lower barrier. Note that comparison with the continuum model is less relevant as the barrier for the SiO-Si bond formation in the continuum model was unrealisticly high due to erroneous loss of intramolecular hydrogen bonds. For all three reactions studied, the change from continuum to explicit solvent model yielded a significant lowering of the overall barrier (∼20 kJ/mol). The mechanistic differences between SiO-Si bond formation for linear and cyclic oligomers in explicit water are now responsible for their differences in rate, whereas in the continuum solvent model, it was attributed to hydrogen bond effects. The difference in free energy between cyclic and linear oligomers is less than the enthalpy difference computed in the gas phase, reflecting less destabilization by loss in hydrogen bonding. The strain energy of the cyclic dimer may explain its relative instability with respect to the linear oligomer. In conclusion, we provided insight in the kinetics, thermodynamics, and mechanism of silica oligomerization by highlevel ab initio molecular dynamics simulation of silicate dimer and trimer formation in bulk aqueous solution. We find that in the water cleavage step two competing proton transfer pathways occur. Our results imply that it is essential to incorporate explicit water molecules in computational studies of silica condensation and sol-gel chemistry. Acknowledgment. This work was sponsored by the National Computing Facilities Foundation, which provided supercomputer facilities, with financial support from The Netherlands Organization for Scientific Research (NWO). Support from the National Research School Combination “Catalysis” (NRSCC) is gratefully acknowledged. Supporting Information Available: Computational details of the employed CPMD software package, the initial coordinates
of system, and snapshots of linear trimer and ring trimer formation. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press: Boston, 1990. (2) Iler, R. K. The Chemistry of Silica; John Wiley & Sons: New York, 1979. (3) Cundy, C. S.; Cox, P. A. Chem. ReV. 2003, 103, 663–701. (4) Knight, C. T. G.; Kinrade, S. D. J. Phys. Chem. B 2002, 106, 3329. (5) Kirschhock, C. E. A.; Ravishankar, R.; Verspeurt, F.; Grobet, P. J.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 2002, 106, 3333. (6) Felmy, A. R.; Cho, H.; Rustad, J. R.; Mason, M. J. J. Solution Chem. 2001, 30, 509. (7) de Moor, P.-P. E. A.; Beelen, T. P. M.; van Santen, R. A.; Beck, L. W.; Davis, M. E. J. Phys. Chem. B 2000, 104, 7600. (8) de Moor, P.-P. E. A.; Beelen, T. P. M.; van Santen, R. A.; Tsuji, K.; Davis, M. E. Chem. Mater. 1999, 11, 36. (9) Kirschhock, C. E. A.; Ravishankar, R.; Verspeurt, F.; Grobet, P. J.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4965. (10) Bussian, P.; Sobott, F.; Brutschy, B.; Schrader, W.; Schu¨th, F. Angew. Chem., Int. Ed. 2000, 39, 3901. (11) Pelster, S. A.; Schrader, W.; Schuth, F. J. Am. Chem. Soc. 2006, 128, 4310. (12) Pereira, J. C. G.; Catlow, C. R. A.; Price, G. D. J. Phys. Chem. A. 1999, 103, 3252. (13) Pereira, J. C. G.; Catlow, C. R. A.; Price, G. D. J. Phys. Chem. A. 1999, 103, 3268. (14) Pereira, J. C. G.; Catlow, C. R. A.; Price, G. D. Chem. Commun. 1998, 1387. (15) Tossell, J. A. Geochim. Cosmochim. Acta 2005, 69, 283. (16) Mora-Fonz, M. J.; Catlow, C. R. A.; Lewis, D. W. Angew. Chem. Int. Ed 2005, 44, 3082. (17) Pereira, J. C. G.; Catlow, C. R. A.; Price, G. D.; Almeida, R. M. J. Sol-Gel Sci. Technol. 1997, 8, 55. (18) Rao, N. Z.; Gelb, L. D. J. Phys. Chem B 2004, 108, 12418. (19) Catlow, C. R. A.; Coombes, D. S.; Lewis, D. W.; Pereira, J. C. G. Chem. Mater. 1998, 10, 3249. (20) Trinh, T. T.; Jansen, A. P. J.; van Santen, R. A. J. Phys. Chem. B 2006, 110, 23099. (21) van Erp, T. S.; Meijer, E. J. Angew. Chem., Int. Ed. 2004, 43, 1660. (22) Car, R.; Parrinello, M. Phys. ReV. Lett. 1985, 55, 2471.
2652 J. Phys. Chem. C, Vol. 113, No. 7, 2009 (23) CPMD, version 3.11, developed by Hutter, J.; Ballone, P.; Ballone, P.; Bernasconi, M.; Focher, P. Fois, E. Goedecker, S.; Parrinello, M.; and Tuckermann, M., Copyright IBM Corp 1990-2006, Copyright MPI fu¨r Festko¨rperforschung Stuttgart 1997-2001. (24) See Supporting Information for details. (25) Trouiller, N.; Martins, J. L. Phys. ReV. B. 1991, 43, 1993. (26) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B. 1988, 37, 785. (27) Laasonen, K.; Sprik, M.; Parrinello, M.; Car, R. J. Chem. Phys. 1993, 99, 9080. (28) Sprik, M.; Hutter, J.; Parrinello, M. J. Chem. Phys. 1996, 105, 1142. (29) Silvestrelli, P. L.; Parrinello, M. J. Chem. Phys. 1999, 111, 3572.
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