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Molecular Modelling Studies of Zeolite Synthesis
J. Weitkamp, H.G. Karge, H. Pfeifer and W. Hblderich (Eds.) Zeoliies and Related Microporous Maierials: State of the Ari 1994 Studies in Surface Science and Catalysis, Vol. 84 0 1994 Elsevier Science B.V. All rights reserved.
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Molecular Modelling Studies of Zeolite Synthesis P.A Coxa, A.P. Stevensa, L. Bantinga and A.M. Gormanb aDepartment of Chemistry, University of Portsmouth, St. Michaels Building, White Swan Road, Portsmouth, PO1 2DT, UK. bBiosym Technologies, 9685 Scranton Road, San Diego, CA, USA. ABSTRACT A number of molecular modelling methods have been used to investigate the relationship between zeolite structures and the organic molecules used in their synthesis. The results reveal the remarkable correlation that exists between the shape of the organic molecule and that of the zeolite's pore channel system. These techniques greatly enhance our understanding of zeolite formation, and have clear benefits for directing novel synthesis programmes. 1. INTRODUCTION
The synthesis of novel high-silica zeolites usually involves adding different organic molecules (commonly referred to as 'templates') to synthesis mixtures of silica and alumina. Then, if the researcher is fortunate enough to synthesise a new material, its properties are tested in the hope that it has some useful applications. An attractive alternative mode of development would be to design and synthesise new zeolites which are specifically tailored for a particular reaction. Molecular modelling has the potential to achieve this aim via the following steps:(i) Derivation of potential hypothetical structures, whose viability may be assessed using energy minimisation I free energy calculations. (ii) Modelling the catalytic reaction in order to target a specific hypothetical zeolite structure. (iii) Designing a suitable organic molecule that can be used to synthesise the targeted zeolite. Several papers have been published in the literature on hypothetical structures [ 1,2,3] and modelliig catalytic reactions and sorption by zeolites [see for example 4,5]. In this work we aim to address step 3 above, by studying the relationships between known templateframework pairs in order to help establish rules for template design.
21 16
A study of the relationship between known framework-template pairs is an ideal application for molecular modelling studies because experimental work is hampered by the difficulties posed for x-ray diffraction studies by the powder-like nature of most samples and the problems of locating light atoms within the framework structure. The lock and key analogy widely used in drug design seems applicable to template derivation. Selective binding of a drug to a receptor site is influenced by three major factors; steric, Coulombic and hydrophobic complementarity, with steric fit playing a determining role in binding. Using this assumption, modelling techniques which have been extensively used in drug design, such as shape and electrostatic similarity calculations, visual docking methods and molecular mechanics calculations have been applied to study zeolite-template pairs. In addition, we report a novel automated docking approach based on Monte Carlo and simulated annealing methods.
2. METHOD 2.1 Molecular Mechanics Most of the work reported here centres around the use of the molecular mechanics method for calculating molecular energies. This technique treats molecules as simple mechanical models with hard spheres representing the atoms and springs the bonds. The energy of the system can then be written as a sum of two, three and four body terms for appropriate bonded and non-bonded terms. Such an expression is known as a forcefield and suitable parameters for different atom types can be derived. A typical forcefield contains many terms e.g. :-
where Eb represents bond stretching, Eg bond angle bending, E+ dihedral angle torsion, EX out of plane bending, Eelec electrostatic interactions, Enb non bonded interactions and Ehb hydrogen bonding. From such an expression the optimum geometry of the molecule can be evaluated by minimising the energy of the system with respect to the atomic positions. In this study, the CVFF forcefield incorporated into the Biosym package Discover has been used. 2.2 Shape and electrostatic similarity After applying molecular mechanics procedures to template molecules, the optimised structures can provide the basis for calculation of physical properties. These properties can either be directly compared (e.g. molecular volume, components of the moment of inertia) or, in the case of electrostatic potential or field, via calculation of molecular similarity indices [6,7]. 2.3 Monte Carlo / Simulated Annealing The key advantage of applying this approach in determining template location is that it removes the necessity to dock templates 'by eye' into the zeolite framework. Thus, no prior assumptions about potential docking sites need to be made. In essence, the Monte Carlo procedure puts a guest molecule into the zeolite framework in a random location and orientation and then calculates the interaction energy for the organidzeolite system under investigation in the way described above. If the calculated interaction energy is lower than a
21 17
specified threshold energy then this conformation is accepted. However, if it is higher in energy then the conformation is rejected and another location and orientation are selected until the criteria is satisfied. Using this approach, multiple molecules can be packed into a structure. Simulated annealing techniques are then applied to optimise the locations and conformations for the template molecules by simulating them at high temperatures (i.e. with large amounts of kinetic energy) and then using a combination of slow cooling and the progressive addition of zeolite-template interaction terms in order to obtain low-energy configurations. 3. RESULTS
3.1 Molecular Similarity Extensive molecular mechanics studies, including conformational searching, have been applied in order to optimise seventeen organic template molecules known to synthesise either ZSM-48, EU-I, Nonasil, ZSM-I 1,ZSM-18 or NU-3. These zeolites were chosen in order to give a range of contrasting pore-channel systems. For these template molecules, an analysis of their calculated molecular properties shows that molecules which have similar sizes and shapes form the same zeolite product. This is best exemplified by Figure 1 which shows the strong relationship between the calculated components of the moment of inertia for these templates and their synthesis product. In contrast, based on optimised similarity indices, we observe no strong correlation between the calculated electrostatic field and potentials for different templates that make the same product, suggesting that template geometry plays a dominant role in determining the zeolite product that is formed
6
-1
~
_ _ . ~ .
0 ZSM-48
'
5
*
"
eu-1
NONASlL ZSM-11 ZShl-18 NU-3
4j
RY axis
Figure 1. Plot of x and y components of moments of inertia for templates studied.
Figure 2. Triquat docked in ZSM-18.
2118
3.2 Manual docking A clear insight into why the shape of the organic structure-directing molecule is so closely related to the zeolite structure synthesised can be readily observed using simple molecular graphics techniques, These enable us to 'slice' through the zeolite crystal structure along the appropriate Miller planes in order to observe the detailed nature of the zeolite's pore channel system. The organic molecule can then be docked 'by eye' relative to the channel system. A striking correlation between the geometry of the template molecule and that of the zeolite's channel system is often readily observed. A good example is provided by ZSM-18 and the triquat molecule used in its synthesis [8]. Figure 2 shows triquat docked inside a cross-section through the ZSM-18 structure. We note that the 'triangular' shape of the triquat molecule fits perfectly inside the zeolite's channels. Figure 3 shows the tetrabutylammonium (TBA) template molecule docked inside the channel system of ZSM-11. It can be seen that the channel intersection is the only location for the TBA molecule to be docked into the structure. Again, we observe a clear correlation between the geometry of the TBA molecule and the shape of the zeolite's channel system. The results enable us to develop the idea that the organic molecule causes a specific zeolite structure to form because it forces TO4 to arrange themselves around it in a way which is dependent on the size and shape of the template molecule. Hence void filling and geometric match between zeolite and organic represents an important consideration in template design. Generating a "pore map" of the zeolite's channel system is also a very usefid way of looking in detail at its pore channel system. A pore map contours the van der Waals' surface of the channels without showing the framework atoms themselves. Figure 4 shows the pore map of zeolite NU-87 with the decamethonium ion used in its synthesis shown on the same scale below. The clear match between the length of the bridging 12-ring channel in the zeolite and the length of template molecule enables us to predict that the decamethonium molecule is sited at this location [9].
Figure 3. Two TBA molecules in ZSM-11. Figure 4. NU87 pore map and decamethonium.
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3.3 Monte Carlo I simulated annealing studies Whilst the above approach allows us to study the void filling ability of the rigid template molecule it yields no information of the energetics or the conformation adopted by the organic at a particular location. In this section, we evaluate the combined Monte Carlo / simulated annealing method as a means of determining template location and conformation. Four systems have been studied in detail:* N-methyl quinuclidinium in NU-3 (LEV) type zeolite. This system is a particularly suitable starting point because it is one of the few examples where the template's location has been determined experimentally [lo]. Triquat in ZSM-18. This is an interesting example to look at because it can help to confirm the applicability of the docking work described earlier. Tetrapiperazinium in Nonasil. Several templates may be used to synthesise this structure, some radically different in nature. Work on this system is a prelude to studying other NonasiVTemplate systems. The organic molecule used in the synthesis of the novel structure SSZ-26 [ 113.
SSZ-26 has a novel 10/12-ring channel system. The results of these studies are summarised over the page:-
Figure 5 . Two N-methylquinuclidinium molecules optimised in NU-3.
Figure 6. Two triquat molecules optimised in ZSM-18.
2120
3.3.1 NU-3 A maximum of six N-methyl quinuclidinium templates per unit cell are accommodated in the structure (one per cage). This is in agreement with experiment. Optimisation of these molecules using the simulated annealing approach produces a highly symmetrical arrangement. Figure 5 shows the optimised location for two N-methyl quinuclidinium molecules. All six templates in the unit cell optimise with the methyl group aligned along the zeolite's c-axis. This is again in agreement with experiment. Excellent void filling is achieved. 3.3.2 ZSM-18 Two triquat molecules are accommodated within the unit cell. These optimise with successive templates rotated through 600 with respect to each other, in agreement with the 'manual' docking studies described earlier. Figure 6 shows two successive triquat molecules optimised in ZSM-I 8's pore channel system. 3.3.3 Nonasil A maximum of four templates per unit cell can be accommodated within the nonasil structure. Again, excellent void filling is achieved by the template molecules which we note align themselves identically within each of the zeolite's medium-sized cages (Figure 7). 3.3.4 SSZ-26 Optimisation of the template molecule for this system helps us to understand why an unusual 10 / 12-ring channel system is obtained. The two bulky N+(CH3)3 units align themselves to form the zeolite's 12-ring channel system, whilst the cyclohexane ring is responsible for forming the orthogonal 10-ring channels. Excellent void filling is achieved by the templates, giving us confidence in the predicted locations (Figure 8).
Figure 7. Three tetrapiperazinium molecules optimised in nonasil
2121
Figure 8. Optimised location of template molecules in SSZ-26.
4 CONCLUSIONS
Molecular modelling clearly has a key role to play in helping us to understand the role of organic template molecules in zeolite synthesis. These studies demonstrate the importance of the geometry of the template molecule in determining the type of zeolite structure formed. The techniques described provide valuable tools which may be used to screen templates based on geometric match prior to experimental investigation. Further refinement of these methods will eventually revolutionise our approach to zeolite synthesis, whereby template molecules are designed for desirable targets, thus reducing the dependence on 'good fortune' in novel syntheis work. 5 ACKNOWLEDGEMENTS
PAC would like to thank Biosym Technologies for provision of software under their University Grants Scheme and SERC for financial support.
2122 REFERENCES 1. J.V. Smith, Chem. Rev. 88 (1988)149. 2. D.E. Ackporiaye, Z. Kristallogr., 1.88(1989) 103. 3. S.Ramdas, J. Comp. Aided Mol. Des., 2, (1988)137. 4. C.M. Freeman, C.R.A. Catlow, J.M. Thomas and S. Brode, Chem. Phys. Lett., 186 (1991) 137. 5. C.M. Freeman and C.R.A. Catlow, Chem & Ind., (1990)796. 6. R. Carbo, L. Leyda and M. Arnau, Int. J. Quant. Chem., 17 (1980)1185. 7. E.E. Hodgkin and W.G. Richards, Int. J. Quant. Chem., 14 (1978) 105. 8. S.L. Lawton and W.J. Rohrbaugh, Science, 247 (1990)1319. 9. J.L. Casci and P.A. Cox, in preparation. 10. McCusker L.B., in "Proceedingsof the 8th International Zeolite Conference:Recent Research Reports", ed. Jansen J.C., Moscou L. & Post M.F.M., p281-2, Amsterdam:Elsevier (1989). 11. R.F. Lobo, M. Pan, H-X. Li, R.C. Medrud, S.I. Zones, P.A. Crozier and M.E. Davis, Science, 262 ( 1993) 1 543.
21 15
Molecular Modelling Studies of Zeolite Synthesis P.A Coxa, A.P. Stevensa, L. Bantinga and A.M. Gormanb aDepartment of Chemistry, University of Portsmouth, St. Michaels Building, White Swan Road, Portsmouth, PO1 2DT, UK. bBiosym Technologies, 9685 Scranton Road, San Diego, CA, USA. ABSTRACT A number of molecular modelling methods have been used to investigate the relationship between zeolite structures and the organic molecules used in their synthesis. The results reveal the remarkable correlation that exists between the shape of the organic molecule and that of the zeolite's pore channel system. These techniques greatly enhance our understanding of zeolite formation, and have clear benefits for directing novel synthesis programmes. 1. INTRODUCTION
The synthesis of novel high-silica zeolites usually involves adding different organic molecules (commonly referred to as 'templates') to synthesis mixtures of silica and alumina. Then, if the researcher is fortunate enough to synthesise a new material, its properties are tested in the hope that it has some useful applications. An attractive alternative mode of development would be to design and synthesise new zeolites which are specifically tailored for a particular reaction. Molecular modelling has the potential to achieve this aim via the following steps:(i) Derivation of potential hypothetical structures, whose viability may be assessed using energy minimisation I free energy calculations. (ii) Modelling the catalytic reaction in order to target a specific hypothetical zeolite structure. (iii) Designing a suitable organic molecule that can be used to synthesise the targeted zeolite. Several papers have been published in the literature on hypothetical structures [ 1,2,3] and modelliig catalytic reactions and sorption by zeolites [see for example 4,5]. In this work we aim to address step 3 above, by studying the relationships between known templateframework pairs in order to help establish rules for template design.
21 16
A study of the relationship between known framework-template pairs is an ideal application for molecular modelling studies because experimental work is hampered by the difficulties posed for x-ray diffraction studies by the powder-like nature of most samples and the problems of locating light atoms within the framework structure. The lock and key analogy widely used in drug design seems applicable to template derivation. Selective binding of a drug to a receptor site is influenced by three major factors; steric, Coulombic and hydrophobic complementarity, with steric fit playing a determining role in binding. Using this assumption, modelling techniques which have been extensively used in drug design, such as shape and electrostatic similarity calculations, visual docking methods and molecular mechanics calculations have been applied to study zeolite-template pairs. In addition, we report a novel automated docking approach based on Monte Carlo and simulated annealing methods.
2. METHOD 2.1 Molecular Mechanics Most of the work reported here centres around the use of the molecular mechanics method for calculating molecular energies. This technique treats molecules as simple mechanical models with hard spheres representing the atoms and springs the bonds. The energy of the system can then be written as a sum of two, three and four body terms for appropriate bonded and non-bonded terms. Such an expression is known as a forcefield and suitable parameters for different atom types can be derived. A typical forcefield contains many terms e.g. :-
where Eb represents bond stretching, Eg bond angle bending, E+ dihedral angle torsion, EX out of plane bending, Eelec electrostatic interactions, Enb non bonded interactions and Ehb hydrogen bonding. From such an expression the optimum geometry of the molecule can be evaluated by minimising the energy of the system with respect to the atomic positions. In this study, the CVFF forcefield incorporated into the Biosym package Discover has been used. 2.2 Shape and electrostatic similarity After applying molecular mechanics procedures to template molecules, the optimised structures can provide the basis for calculation of physical properties. These properties can either be directly compared (e.g. molecular volume, components of the moment of inertia) or, in the case of electrostatic potential or field, via calculation of molecular similarity indices [6,7]. 2.3 Monte Carlo / Simulated Annealing The key advantage of applying this approach in determining template location is that it removes the necessity to dock templates 'by eye' into the zeolite framework. Thus, no prior assumptions about potential docking sites need to be made. In essence, the Monte Carlo procedure puts a guest molecule into the zeolite framework in a random location and orientation and then calculates the interaction energy for the organidzeolite system under investigation in the way described above. If the calculated interaction energy is lower than a
21 17
specified threshold energy then this conformation is accepted. However, if it is higher in energy then the conformation is rejected and another location and orientation are selected until the criteria is satisfied. Using this approach, multiple molecules can be packed into a structure. Simulated annealing techniques are then applied to optimise the locations and conformations for the template molecules by simulating them at high temperatures (i.e. with large amounts of kinetic energy) and then using a combination of slow cooling and the progressive addition of zeolite-template interaction terms in order to obtain low-energy configurations. 3. RESULTS
3.1 Molecular Similarity Extensive molecular mechanics studies, including conformational searching, have been applied in order to optimise seventeen organic template molecules known to synthesise either ZSM-48, EU-I, Nonasil, ZSM-I 1,ZSM-18 or NU-3. These zeolites were chosen in order to give a range of contrasting pore-channel systems. For these template molecules, an analysis of their calculated molecular properties shows that molecules which have similar sizes and shapes form the same zeolite product. This is best exemplified by Figure 1 which shows the strong relationship between the calculated components of the moment of inertia for these templates and their synthesis product. In contrast, based on optimised similarity indices, we observe no strong correlation between the calculated electrostatic field and potentials for different templates that make the same product, suggesting that template geometry plays a dominant role in determining the zeolite product that is formed
6
-1
~
_ _ . ~ .
0 ZSM-48
'
5
*
"
eu-1
NONASlL ZSM-11 ZShl-18 NU-3
4j
RY axis
Figure 1. Plot of x and y components of moments of inertia for templates studied.
Figure 2. Triquat docked in ZSM-18.
2118
3.2 Manual docking A clear insight into why the shape of the organic structure-directing molecule is so closely related to the zeolite structure synthesised can be readily observed using simple molecular graphics techniques, These enable us to 'slice' through the zeolite crystal structure along the appropriate Miller planes in order to observe the detailed nature of the zeolite's pore channel system. The organic molecule can then be docked 'by eye' relative to the channel system. A striking correlation between the geometry of the template molecule and that of the zeolite's channel system is often readily observed. A good example is provided by ZSM-18 and the triquat molecule used in its synthesis [8]. Figure 2 shows triquat docked inside a cross-section through the ZSM-18 structure. We note that the 'triangular' shape of the triquat molecule fits perfectly inside the zeolite's channels. Figure 3 shows the tetrabutylammonium (TBA) template molecule docked inside the channel system of ZSM-11. It can be seen that the channel intersection is the only location for the TBA molecule to be docked into the structure. Again, we observe a clear correlation between the geometry of the TBA molecule and the shape of the zeolite's channel system. The results enable us to develop the idea that the organic molecule causes a specific zeolite structure to form because it forces TO4 to arrange themselves around it in a way which is dependent on the size and shape of the template molecule. Hence void filling and geometric match between zeolite and organic represents an important consideration in template design. Generating a "pore map" of the zeolite's channel system is also a very usefid way of looking in detail at its pore channel system. A pore map contours the van der Waals' surface of the channels without showing the framework atoms themselves. Figure 4 shows the pore map of zeolite NU-87 with the decamethonium ion used in its synthesis shown on the same scale below. The clear match between the length of the bridging 12-ring channel in the zeolite and the length of template molecule enables us to predict that the decamethonium molecule is sited at this location [9].
Figure 3. Two TBA molecules in ZSM-11. Figure 4. NU87 pore map and decamethonium.
21 19
3.3 Monte Carlo I simulated annealing studies Whilst the above approach allows us to study the void filling ability of the rigid template molecule it yields no information of the energetics or the conformation adopted by the organic at a particular location. In this section, we evaluate the combined Monte Carlo / simulated annealing method as a means of determining template location and conformation. Four systems have been studied in detail:* N-methyl quinuclidinium in NU-3 (LEV) type zeolite. This system is a particularly suitable starting point because it is one of the few examples where the template's location has been determined experimentally [lo]. Triquat in ZSM-18. This is an interesting example to look at because it can help to confirm the applicability of the docking work described earlier. Tetrapiperazinium in Nonasil. Several templates may be used to synthesise this structure, some radically different in nature. Work on this system is a prelude to studying other NonasiVTemplate systems. The organic molecule used in the synthesis of the novel structure SSZ-26 [ 113.
SSZ-26 has a novel 10/12-ring channel system. The results of these studies are summarised over the page:-
Figure 5 . Two N-methylquinuclidinium molecules optimised in NU-3.
Figure 6. Two triquat molecules optimised in ZSM-18.
2120
3.3.1 NU-3 A maximum of six N-methyl quinuclidinium templates per unit cell are accommodated in the structure (one per cage). This is in agreement with experiment. Optimisation of these molecules using the simulated annealing approach produces a highly symmetrical arrangement. Figure 5 shows the optimised location for two N-methyl quinuclidinium molecules. All six templates in the unit cell optimise with the methyl group aligned along the zeolite's c-axis. This is again in agreement with experiment. Excellent void filling is achieved. 3.3.2 ZSM-18 Two triquat molecules are accommodated within the unit cell. These optimise with successive templates rotated through 600 with respect to each other, in agreement with the 'manual' docking studies described earlier. Figure 6 shows two successive triquat molecules optimised in ZSM-I 8's pore channel system. 3.3.3 Nonasil A maximum of four templates per unit cell can be accommodated within the nonasil structure. Again, excellent void filling is achieved by the template molecules which we note align themselves identically within each of the zeolite's medium-sized cages (Figure 7). 3.3.4 SSZ-26 Optimisation of the template molecule for this system helps us to understand why an unusual 10 / 12-ring channel system is obtained. The two bulky N+(CH3)3 units align themselves to form the zeolite's 12-ring channel system, whilst the cyclohexane ring is responsible for forming the orthogonal 10-ring channels. Excellent void filling is achieved by the templates, giving us confidence in the predicted locations (Figure 8).
Figure 7. Three tetrapiperazinium molecules optimised in nonasil
2121
Figure 8. Optimised location of template molecules in SSZ-26.
4 CONCLUSIONS
Molecular modelling clearly has a key role to play in helping us to understand the role of organic template molecules in zeolite synthesis. These studies demonstrate the importance of the geometry of the template molecule in determining the type of zeolite structure formed. The techniques described provide valuable tools which may be used to screen templates based on geometric match prior to experimental investigation. Further refinement of these methods will eventually revolutionise our approach to zeolite synthesis, whereby template molecules are designed for desirable targets, thus reducing the dependence on 'good fortune' in novel syntheis work. 5 ACKNOWLEDGEMENTS
PAC would like to thank Biosym Technologies for provision of software under their University Grants Scheme and SERC for financial support.
2122 REFERENCES 1. J.V. Smith, Chem. Rev. 88 (1988)149. 2. D.E. Ackporiaye, Z. Kristallogr., 1.88(1989) 103. 3. S.Ramdas, J. Comp. Aided Mol. Des., 2, (1988)137. 4. C.M. Freeman, C.R.A. Catlow, J.M. Thomas and S. Brode, Chem. Phys. Lett., 186 (1991) 137. 5. C.M. Freeman and C.R.A. Catlow, Chem & Ind., (1990)796. 6. R. Carbo, L. Leyda and M. Arnau, Int. J. Quant. Chem., 17 (1980)1185. 7. E.E. Hodgkin and W.G. Richards, Int. J. Quant. Chem., 14 (1978) 105. 8. S.L. Lawton and W.J. Rohrbaugh, Science, 247 (1990)1319. 9. J.L. Casci and P.A. Cox, in preparation. 10. McCusker L.B., in "Proceedingsof the 8th International Zeolite Conference:Recent Research Reports", ed. Jansen J.C., Moscou L. & Post M.F.M., p281-2, Amsterdam:Elsevier (1989). 11. R.F. Lobo, M. Pan, H-X. Li, R.C. Medrud, S.I. Zones, P.A. Crozier and M.E. Davis, Science, 262 ( 1993) 1 543.