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Structure descriptors for organic templates employed in zeolite synthesis
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved9
117
STRUCTURE DESCRIPTORS FOR ORGANIC TEMPLATES EMPLOYED IN Z E O L I T E S Y N T H E S I S R E Boyett,* a,b A P Stevens, a,b M G Ford b and P A Cox a,b Division of Chemistry, University of Portsmouth, St. Michaels Building, White Swan Road, Portsmouth, Hants, PO 1 2DT, UK. b Centre for Molecular Design, University of Portsmouth, Halpem House, 1/2 Hampshire Terrace, Portsmouth, Hants, PO 1 2QF, UK. a
ABSTRACT
A structural analysis has been carried out upon 130 organic templating agents employed in the synthesis of zeolites from 18 different framework types. The orthogonal principal axes of inertia of these molecules provide quantitative structure descriptors which can be plotted to produce three-dimensional 'shape-space' diagrams. Groups of templates which produce different zeolite framework architectures plot in discrete areas of these graphs. If the set of templates for a single framework type occupy more than one region of this shape-space, it can indicate that several modes of incorporation are found for the template molecules within the relevant zeolite porespace. Large, complex templates can often be demonstrated to comprise multiple structural units, or sub-shapes, the latter having similar dimensions to the smallest agents in any one set of templates, i.e. those that fulfil the minimum void-filling requirements of particular pores. Regions of overlap may occur between whole families of templates in the principal axes of inertia shape-space. The templates which plot within an overlap region may direct the formation of any of the different zeolite frameworks whose template shape-space envelopes interpenetrate at that point. The actual outcome of syntheses that deploy templates from these regions thereby depend more critically than usual upon factors besides just template shape, such as the composition of the reaction media and the reaction conditions. The quantitative description of template size and shape afforded by the use of principal axes of inertia highlights the importance of template shape in determining the zeolite product formed.
1. INTRODUCTION
Zeolite catalysts are being synthesised to an increasing extent via the use of gels that contain specific organic templating or structure directing agents. The exact r61e which these agents play in zeolite formation has not been established, but previous research has shown the size and the shape of any particular template molecule to be an important factor in defining the lattice structure of the eventual zeolite product, t1'2'3'4]as, indeed, is the template orientation. [51 9
118 Although a number of molecular modelling techniques have been applied in zeolite research, [6'7~none have yet provided a tool facilitating the quantitative description of template structure. In order to address this need we have investigated techniques that have a proven record in the disciplines of Quantitative Structure-Activity Relationships (QSAR), I8~ and drug design, applying the lock and key analog)Jg~ for the conformational interaction between drug and receptor site in a novel way, to rationalise the relationship between the structures of template molecules which direct the formation of the same products. Several different properties are suitable for the task of structure representation, t1~ the present study has employed the molecular principal axes of inertia. The results discussed below identify such descriptors as potential aids to our understanding of template-framework relationships, and we discuss their potential for application in the rational design of novel materials.
2. E X P E R I M E N T A L
Structure analysis was carried out upon 130 organic templating agents which have been employed in 229 syntheses documented in research literature and in patent applications. [1il The zeolites formed using these 130 agents exhibit 18 different framework structures; all examples investigated here were of a siliceous or aluminosilicate composition, and our conclusions are therefore restricted to zeolite-template systems of this type. Molecular mechanics calculations were used to defme a low potential energy conformation for each molecule, utilising the potentials and parameters in the CVFF force field, implemented within the program Discover. c12! The orthogonal principal axes of inertia, RN, were calculated using the Tsar Quantitative Structure-Activity Relationships (QSAR) software, t13~ These quantities are customarily described in ,~mgstroms, and define an ellipsoid (see Figure 1) which is scaled in inverse proportion to the molecular moments of inertia for a given template. RY
cr---
~
i
~
- - ~ '
RX
Figure 1. Inertial ellipsoid for a typical template molecule. The dimensions of the ellipsoid, RN, are inversely proportional to the molecular moments of inertia, IN. Two assumptions have been made in order to apply the principal axes of inertia in this way. Firstly, the derived molecular conformations are taken to represent those of templates located inside pore systems, without the explicit consideration of either thermal motion or of any conformational change of the template within the host arising from lattice-template interaction. Secondly, the masses of the C, N, and O atoms defining the 'backbones' of the templates are considered to be similar enough to permit a direct comparison of moments, and therefore RN values, between all molecules in the survey. These assumptions do not affect the employment of principal axes of inertia as a tool to describe the structure of templates at a first
119 approximation, indeed, the axes retain inherent advantages over alternative shape descriptors, being convenient to calculate, and requiring no pre-alignment, or definition of reference atoms.
3. RESULTS AND DISCUSSION The use of a template during zeolite synthesis can impart a very strong influence on the pore architecture developed within an eventual product. The extent to which the influence is controlled by the shape of the template is illustrated in the three-dimensional plots below, which depict the principal axes of inertia for several systems. The templates making different products are found to occupy discrete RN 'shape-space.' It should be noted that a little distinction is lost owing to the reduction of the 3-D plot onto a plane projection, and that only a limited groups of structures may be included in any one plot without significant loss of clarity. The data shown in Figure 2 represent six different framework types in which the templates are enclosed in clearly defined shape-space, this degree of enclosure may be rationalised quite simply. For example, the templates that form the cage-like cavities in NON-type zeolites are small molecules whose structures essentially occupy a spherical or toroid volume, agents such as adamantanes and unsaturated ring compounds, respectively. These molecules share similar RN values, where all three components are of approximately the same magnitude. The other framework types depicted in the figure are also shape specific in their templating requirements. Docking studies t4] indicate that only molecules of very precise dimensions may locate within the pockets along the pores of zeolite EU-1 (EUO). Similarly, the templates found to direct the crystallisation of zeolites ZSM-18 (MEI) and ZSM-57 (MFS) are seemingly very few, and occupy small, specific volumes of shape-space on the graphs. This is arguably because only very few compounds have the correct size and shape to direct the formation of these particular pore architectures. 4 3.6 3.2 2.8
i
11a
I ! I
2.4
m
ml
m
m
I
ml
m
1.2
0
2 RY
20 4
6 ~
80 lo lOO
60
40 RX
Figure 2. Principle axes of inertia (]k) for templates that produce zeolites assigned to the framework categories" NON ~ , MEL IlL EUO O, LEV &, MEI 1r and MFS ~.
120
All but one of the agents represented in Figure 2 and which form the MEL-type structure are homologous linear diamines which exhibit closely related RY and RZ values but have large and variable RX components, such that they plot as a parallel band in the figure. These templates are encapsulated along straight pore channels in ZSM-11 and its analogues. In contrast, tetrabutylammonium, whose RX and RY values are identical but whose RY and RZ values are comparatively larger than the diamines, is located centrally at pore intersection sites. The principal axes of inertia can clearly distinguish between these different modes of template accomodation within one pore system, as well as between framework varieties, evidence for such modes arising from the division of the shape-space for a given template set into subregions, one being found for each mode.
2.4 2.2 2
1.8 1.6 1.4 1.:
I
100
80
ou
--
RX
Figure 3. Principle axes of inertia (/~) of templates making up zeolites with MTT frameworks. Templates are coded thus: ,ik basic unit, occupying a single lobe; Ill double unit, occupying two lobes; and 9 multiple lobe-occupying units. Table 1. Maximum and minimum values for principal axes of inertia of templates which form zeolites with a MTT type framework.
Lobes occupied
Value
R X / / ~ RY / A R Z / A
Surface Molecular Area/]k z V o l u m e / A 3
Single
Max Min Only Max Min
11.9 2.7 25.8 97.0 39.0
108 82 167 378 290
Double Multiple
2.70 1.47 1.50 2.24 2.21
1.56 1.31 1.46 2.23 2.21
63 44 97 269 203
Unidimensional zeolite pore systems may exhibit several modes: there are at least three ways in which templates can be incorporated within the pores of zeolites which have the MTT-
121 type pore system, (e.g. ZSM-23). In addition, this framework type can also be used to introduce the concept of template shape and 'sub-shape.' The five smallest templates represented in Figure 3 and Table 1 can effectively be considered as void-filling traits ('mode 1') whose shape and molecular volume, c. 50 A, define the most basic requirements for the unidimensional MTT pore channels, with their regularly spaced, undulating lobes. Figure 4 illustrates the relationship of two small templates to these pore lobes, the organic agents pyrrolidine and 2-aminoethanol having been docked into the channels manually using computer graphics. The poremap in this and other similar figures has been defined by showing the space accessible to a template after van der Waals surfaces have been generated around the zeolite's framework. The other templates in the MTT set occupy more than one pore lobe, either by spanning two adjacent lobes (dipropylamine, 'mode 2') or by bridging between more distant cavities (as do the diammonium ions, 'mode 3'). The basic units such as pyrrolidine may therefore be envisaged as effective sub-shapes of the larger compounds. The four diammonium compounds typify a feature frequently seen amongst sets of templates in our investigation, in that they comprise a co/nmon chemical form with repeated structural units, and their total volume is approximately a multiple of that found for the basic void-filling templates, (Table 1). Only heptamethonium is shown within the pore in the figure, although presumably the other agents, octa-, undeca- and dodecamethonium, are accomodated in a closely-related fashion, with their quarternary nitrogen centres housed in pore lobes. It is not clear from Figure 4 whether nona- and decamethonium are unable to produce MTT systems in this way, or whether they have .been used successfully to synthesise such structures but were simply not identified in our primary search of the literature. If the lobe spacing does restrict the alkyl chains linking the nitrogens in diammoniums to certain permitted lengths, it appears from the figure and table that another pair of templating agents may be available with RX values of approximately 140 A, and molecular volumes of around 310 A 3.
I
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........." ~.
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{.::::~::;;~:::;~:::.:i..::==========================:~::::~..:::.i..::~:::::~::::~:::..~.:::~i:;;~;::;~:::.:i.:::~;::::~:;::~:::.:~
Figure 4. Poremap showing the lobed one-dimensional channel system found in zeolites with the MTT structure. Manually docked molecules illustrate possible sites available within the lobes, from top left: pyrrolidine; 2-aminoethanol; dipropylamine; and heptamethonium.
122 Plotting the principal axes of inertia for a particular template can give some indication as to which zeolite framework structures may be synthesised using that agent. The dibenzyldimethylammonium ion (DBDMA) forms at least three types of zeolite lattice (see Figure 5), the actual outcome of a preparation being dependant not only on the template shape, but also on the chemical composition of the reaction media, and the conditions under which the synthesis is performed. The DBDMA ion can therefore be considered to be a member of three distinct sets of templates. The principal axes of inertia for these templates are plotted in Figure 6; DBDMA occupies a point which lies at the intersection of the three regions of 'shape-space' relating to the structure types BEA, EUO, and MTW. The ion is accomodated in different ways in each lattice, as illustrated in Figure 5.
a
b
C
Figure 5. Three different modes of accomodation for dibenzyldimethylammonium in zeolite framework types a) BEA, b) EUO, and c) MTW. The overlap region between sets of template structures becomes very pronounced towards the origin of the RX plots. In examples such as the NON, SGT, MTN, DDR, and DOH frameworks, factors other than RX values may also need to be considered in order to differentiate between what are actually very closely related sets of templates. The small template molecules that plot near the origin can, like pyrrolidine, produce a variety of structures, depending on a range of other variables in any given synthesis. Other shape descriptors from QSAR could yet be suitable for template analysis, and although none might have the convenience characteristic of the principal axes of inertia, they
123
3
x
2.8 2.6
.,,
=// =1://,
"1= l/l/
2
1.4
RY
3 4
70 60 50 40 30 20 10 0 RX 5
80
Figure 6. Principal axes of inertia (/~) for templates forming BEA it, EUO m, and MTW (part of set) ~ framework types. DBDMA O, falls at the intersection of the three types. may provide additional structural information. Comparative Molecular Field Analysis [14'151 (CoMFA) may be used to investigate the steric and electrostatic fields which define templates. However, CoMFA requires that target molecules be aligned identically before any comparison ac be made, and therefore analyses are restricted only to very closely related sets of templating agents. It may however provide a tool which can identify the template charge characteristics necessary to produce zeolites with a certain Si / A1 ratio, or crystal morphology. It has been suggested that CoMFA be applied directly to describe the quantitative structure-activity relationships of zeolites with known catalytic properties, t141
4. CONCLUSIONS The principal axes of inertia provide a basic quantitative description of the shape of organic templating agents, and if taken into account with molecular volumes, can increase the current understanding of the influence of template structure on the structure of zeolite pore systems. These descriptors also offer an indication as to the differing manners by which various templates are incorporated into the channel system of any one zeolite, t16] The use of structure descriptors is complementary to Monte Carlo-Simulated Annealing [4'171 and, perhaps, D e N o v o methods, and could establish which shape criteria are necessary to allow rational computer assisted design of novel catalytic zeolite systems to be realised. New and improved templating agents could be sought by screening the RN values of candidate molecules, and comparing them against the known values from previously successful templates. Novel templates may also be pursued using the concept of shape and sub-shape, whereby, for instance, cheaper and smaller agents may be identified as effective sub-units of larger molecules, and vice versa. Plotting a new template upon a principal axes of inertia 'shape-space diagram' may predict which zeolite structures it might produce in addition to any
124 (more desirable) target type, and then the optimum reaction conditions necessary for its deployment may begin to be established. The authors are currently seeking to supplement the described preliminary survey by accounting for the importance of template shape in respect to other variables in zeolite synthesis, notably the electrostatic profiles of organic templates, gel chemistry and reaction conditions. A detailed chemometric or quantitative structure-property relationships (QSPR) analysis of the available data may yield information which will permit specific preparative methods to be identified for the deployment of novel templating agents.
REFERENCES
10
11
12 13 14 15 16 17
H. Gies and B. Marler, Zeolites, 12, (1992), 42. B.M. Lok, T.R. Cannan, and C.A. Messina, Zeolites, 3, (1983), 282. L.B. McCusker, Materials Science Forum, 133-136, (1993), 423. A.P. Stevens, A.M. Gorman, C.M. Freeman, and P.A. Cox, submitted to J. Chem. Soc., Faraday Trans., (November 1995). A.P. Stevens and P.A. Cox, J. Chem. Soc., Chem. Commun., (1995), 343. See, for example, "Modelling of Structure and Reactivity in Zeolites," (ed. C.R.A. Catlow), Academic Press, London, (1992). C.M. Freeman, D.W. Lewis, T.V. Harris, A.K. Cheetham, N.J. Henson, P.A. Cox, A.M. Gorman, S.M. Levine, J.M. Newsam, E. Hemandez, and C.R.A. Catlow, in "ComputerAided Molecular Design: Agrochemicals, Materials and Pharmaceuticals," (eds. C.H. Reynolds, M.K. Holloway, and H.K. Cox), ACS, Washington DC, (1995), 326. D. Livingstone, "Data Analysis for Chemists," Oxford University Press, (1995). P.M. Dean, "Molecular Foundations of Drug-Receptor Interaction," Cambridge University Press, (1987), 254. P.C. Jurs, S.L. Dixon, and L.M. Egolf, in "Chemometric Methods' in Molecular Design, Methods' and Principles in Medicinal Chemistry Volume 2," (ed. H. van de Waterbeemd), VCH, (1995), 15. A list of calculated values for the principal axes of inertia relating to the templates included in this study are available on request from the authors, together with full details of the literature source for each. Discover 3.1 program, Insight H User Guide, Version 2.3.6, Biosym Technologies Inc., San Diego, USA, (1993). Tsar version 2.02, Oxford Molecular Ltd., UK, (1993). R.D. Cramer IU, D.E. Patterson, and J.D. Bunce, J. Am. Chem. Soc., 110, (1988), 5959. K.H. Kim, in "Molecular Similarity in Drug Design," (ed. P.M. Dean), Blackie Academic and Professional, Glasgow, (1995), 291. See also R.E. Boyett, A.P. Stevens, M.G. Ford, and P.A. Cox, to be submitted to Zeolites, 1996. P.A. Cox, A.P. Stevens, L. Banting, and A.M. Gorman, in "Zeolites and Related Microporous Materials: State of the Art 1994," Studies in Surface Science and Catalysis, Vol 84, (eds. J. Weitkamp, H.G. Karge, H. Pfeifer, and W. H/51derich), Elsevier, Science B.V., (1994), 2115.
117
STRUCTURE DESCRIPTORS FOR ORGANIC TEMPLATES EMPLOYED IN Z E O L I T E S Y N T H E S I S R E Boyett,* a,b A P Stevens, a,b M G Ford b and P A Cox a,b Division of Chemistry, University of Portsmouth, St. Michaels Building, White Swan Road, Portsmouth, Hants, PO 1 2DT, UK. b Centre for Molecular Design, University of Portsmouth, Halpem House, 1/2 Hampshire Terrace, Portsmouth, Hants, PO 1 2QF, UK. a
ABSTRACT
A structural analysis has been carried out upon 130 organic templating agents employed in the synthesis of zeolites from 18 different framework types. The orthogonal principal axes of inertia of these molecules provide quantitative structure descriptors which can be plotted to produce three-dimensional 'shape-space' diagrams. Groups of templates which produce different zeolite framework architectures plot in discrete areas of these graphs. If the set of templates for a single framework type occupy more than one region of this shape-space, it can indicate that several modes of incorporation are found for the template molecules within the relevant zeolite porespace. Large, complex templates can often be demonstrated to comprise multiple structural units, or sub-shapes, the latter having similar dimensions to the smallest agents in any one set of templates, i.e. those that fulfil the minimum void-filling requirements of particular pores. Regions of overlap may occur between whole families of templates in the principal axes of inertia shape-space. The templates which plot within an overlap region may direct the formation of any of the different zeolite frameworks whose template shape-space envelopes interpenetrate at that point. The actual outcome of syntheses that deploy templates from these regions thereby depend more critically than usual upon factors besides just template shape, such as the composition of the reaction media and the reaction conditions. The quantitative description of template size and shape afforded by the use of principal axes of inertia highlights the importance of template shape in determining the zeolite product formed.
1. INTRODUCTION
Zeolite catalysts are being synthesised to an increasing extent via the use of gels that contain specific organic templating or structure directing agents. The exact r61e which these agents play in zeolite formation has not been established, but previous research has shown the size and the shape of any particular template molecule to be an important factor in defining the lattice structure of the eventual zeolite product, t1'2'3'4]as, indeed, is the template orientation. [51 9
118 Although a number of molecular modelling techniques have been applied in zeolite research, [6'7~none have yet provided a tool facilitating the quantitative description of template structure. In order to address this need we have investigated techniques that have a proven record in the disciplines of Quantitative Structure-Activity Relationships (QSAR), I8~ and drug design, applying the lock and key analog)Jg~ for the conformational interaction between drug and receptor site in a novel way, to rationalise the relationship between the structures of template molecules which direct the formation of the same products. Several different properties are suitable for the task of structure representation, t1~ the present study has employed the molecular principal axes of inertia. The results discussed below identify such descriptors as potential aids to our understanding of template-framework relationships, and we discuss their potential for application in the rational design of novel materials.
2. E X P E R I M E N T A L
Structure analysis was carried out upon 130 organic templating agents which have been employed in 229 syntheses documented in research literature and in patent applications. [1il The zeolites formed using these 130 agents exhibit 18 different framework structures; all examples investigated here were of a siliceous or aluminosilicate composition, and our conclusions are therefore restricted to zeolite-template systems of this type. Molecular mechanics calculations were used to defme a low potential energy conformation for each molecule, utilising the potentials and parameters in the CVFF force field, implemented within the program Discover. c12! The orthogonal principal axes of inertia, RN, were calculated using the Tsar Quantitative Structure-Activity Relationships (QSAR) software, t13~ These quantities are customarily described in ,~mgstroms, and define an ellipsoid (see Figure 1) which is scaled in inverse proportion to the molecular moments of inertia for a given template. RY
cr---
~
i
~
- - ~ '
RX
Figure 1. Inertial ellipsoid for a typical template molecule. The dimensions of the ellipsoid, RN, are inversely proportional to the molecular moments of inertia, IN. Two assumptions have been made in order to apply the principal axes of inertia in this way. Firstly, the derived molecular conformations are taken to represent those of templates located inside pore systems, without the explicit consideration of either thermal motion or of any conformational change of the template within the host arising from lattice-template interaction. Secondly, the masses of the C, N, and O atoms defining the 'backbones' of the templates are considered to be similar enough to permit a direct comparison of moments, and therefore RN values, between all molecules in the survey. These assumptions do not affect the employment of principal axes of inertia as a tool to describe the structure of templates at a first
119 approximation, indeed, the axes retain inherent advantages over alternative shape descriptors, being convenient to calculate, and requiring no pre-alignment, or definition of reference atoms.
3. RESULTS AND DISCUSSION The use of a template during zeolite synthesis can impart a very strong influence on the pore architecture developed within an eventual product. The extent to which the influence is controlled by the shape of the template is illustrated in the three-dimensional plots below, which depict the principal axes of inertia for several systems. The templates making different products are found to occupy discrete RN 'shape-space.' It should be noted that a little distinction is lost owing to the reduction of the 3-D plot onto a plane projection, and that only a limited groups of structures may be included in any one plot without significant loss of clarity. The data shown in Figure 2 represent six different framework types in which the templates are enclosed in clearly defined shape-space, this degree of enclosure may be rationalised quite simply. For example, the templates that form the cage-like cavities in NON-type zeolites are small molecules whose structures essentially occupy a spherical or toroid volume, agents such as adamantanes and unsaturated ring compounds, respectively. These molecules share similar RN values, where all three components are of approximately the same magnitude. The other framework types depicted in the figure are also shape specific in their templating requirements. Docking studies t4] indicate that only molecules of very precise dimensions may locate within the pockets along the pores of zeolite EU-1 (EUO). Similarly, the templates found to direct the crystallisation of zeolites ZSM-18 (MEI) and ZSM-57 (MFS) are seemingly very few, and occupy small, specific volumes of shape-space on the graphs. This is arguably because only very few compounds have the correct size and shape to direct the formation of these particular pore architectures. 4 3.6 3.2 2.8
i
11a
I ! I
2.4
m
ml
m
m
I
ml
m
1.2
0
2 RY
20 4
6 ~
80 lo lOO
60
40 RX
Figure 2. Principle axes of inertia (]k) for templates that produce zeolites assigned to the framework categories" NON ~ , MEL IlL EUO O, LEV &, MEI 1r and MFS ~.
120
All but one of the agents represented in Figure 2 and which form the MEL-type structure are homologous linear diamines which exhibit closely related RY and RZ values but have large and variable RX components, such that they plot as a parallel band in the figure. These templates are encapsulated along straight pore channels in ZSM-11 and its analogues. In contrast, tetrabutylammonium, whose RX and RY values are identical but whose RY and RZ values are comparatively larger than the diamines, is located centrally at pore intersection sites. The principal axes of inertia can clearly distinguish between these different modes of template accomodation within one pore system, as well as between framework varieties, evidence for such modes arising from the division of the shape-space for a given template set into subregions, one being found for each mode.
2.4 2.2 2
1.8 1.6 1.4 1.:
I
100
80
ou
--
RX
Figure 3. Principle axes of inertia (/~) of templates making up zeolites with MTT frameworks. Templates are coded thus: ,ik basic unit, occupying a single lobe; Ill double unit, occupying two lobes; and 9 multiple lobe-occupying units. Table 1. Maximum and minimum values for principal axes of inertia of templates which form zeolites with a MTT type framework.
Lobes occupied
Value
R X / / ~ RY / A R Z / A
Surface Molecular Area/]k z V o l u m e / A 3
Single
Max Min Only Max Min
11.9 2.7 25.8 97.0 39.0
108 82 167 378 290
Double Multiple
2.70 1.47 1.50 2.24 2.21
1.56 1.31 1.46 2.23 2.21
63 44 97 269 203
Unidimensional zeolite pore systems may exhibit several modes: there are at least three ways in which templates can be incorporated within the pores of zeolites which have the MTT-
121 type pore system, (e.g. ZSM-23). In addition, this framework type can also be used to introduce the concept of template shape and 'sub-shape.' The five smallest templates represented in Figure 3 and Table 1 can effectively be considered as void-filling traits ('mode 1') whose shape and molecular volume, c. 50 A, define the most basic requirements for the unidimensional MTT pore channels, with their regularly spaced, undulating lobes. Figure 4 illustrates the relationship of two small templates to these pore lobes, the organic agents pyrrolidine and 2-aminoethanol having been docked into the channels manually using computer graphics. The poremap in this and other similar figures has been defined by showing the space accessible to a template after van der Waals surfaces have been generated around the zeolite's framework. The other templates in the MTT set occupy more than one pore lobe, either by spanning two adjacent lobes (dipropylamine, 'mode 2') or by bridging between more distant cavities (as do the diammonium ions, 'mode 3'). The basic units such as pyrrolidine may therefore be envisaged as effective sub-shapes of the larger compounds. The four diammonium compounds typify a feature frequently seen amongst sets of templates in our investigation, in that they comprise a co/nmon chemical form with repeated structural units, and their total volume is approximately a multiple of that found for the basic void-filling templates, (Table 1). Only heptamethonium is shown within the pore in the figure, although presumably the other agents, octa-, undeca- and dodecamethonium, are accomodated in a closely-related fashion, with their quarternary nitrogen centres housed in pore lobes. It is not clear from Figure 4 whether nona- and decamethonium are unable to produce MTT systems in this way, or whether they have .been used successfully to synthesise such structures but were simply not identified in our primary search of the literature. If the lobe spacing does restrict the alkyl chains linking the nitrogens in diammoniums to certain permitted lengths, it appears from the figure and table that another pair of templating agents may be available with RX values of approximately 140 A, and molecular volumes of around 310 A 3.
I
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....:~" ",. ...I" .....-~" \ i~/ I ...-.:.. < ..................
........." ~.
~ I
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.,
9
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.~i . . . . ......I ....
~.i
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!
!
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........ I
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9
.....i
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{.::::~::;;~:::;~:::.:i..::==========================:~::::~..:::.i..::~:::::~::::~:::..~.:::~i:;;~;::;~:::.:i.:::~;::::~:;::~:::.:~
Figure 4. Poremap showing the lobed one-dimensional channel system found in zeolites with the MTT structure. Manually docked molecules illustrate possible sites available within the lobes, from top left: pyrrolidine; 2-aminoethanol; dipropylamine; and heptamethonium.
122 Plotting the principal axes of inertia for a particular template can give some indication as to which zeolite framework structures may be synthesised using that agent. The dibenzyldimethylammonium ion (DBDMA) forms at least three types of zeolite lattice (see Figure 5), the actual outcome of a preparation being dependant not only on the template shape, but also on the chemical composition of the reaction media, and the conditions under which the synthesis is performed. The DBDMA ion can therefore be considered to be a member of three distinct sets of templates. The principal axes of inertia for these templates are plotted in Figure 6; DBDMA occupies a point which lies at the intersection of the three regions of 'shape-space' relating to the structure types BEA, EUO, and MTW. The ion is accomodated in different ways in each lattice, as illustrated in Figure 5.
a
b
C
Figure 5. Three different modes of accomodation for dibenzyldimethylammonium in zeolite framework types a) BEA, b) EUO, and c) MTW. The overlap region between sets of template structures becomes very pronounced towards the origin of the RX plots. In examples such as the NON, SGT, MTN, DDR, and DOH frameworks, factors other than RX values may also need to be considered in order to differentiate between what are actually very closely related sets of templates. The small template molecules that plot near the origin can, like pyrrolidine, produce a variety of structures, depending on a range of other variables in any given synthesis. Other shape descriptors from QSAR could yet be suitable for template analysis, and although none might have the convenience characteristic of the principal axes of inertia, they
123
3
x
2.8 2.6
.,,
=// =1://,
"1= l/l/
2
1.4
RY
3 4
70 60 50 40 30 20 10 0 RX 5
80
Figure 6. Principal axes of inertia (/~) for templates forming BEA it, EUO m, and MTW (part of set) ~ framework types. DBDMA O, falls at the intersection of the three types. may provide additional structural information. Comparative Molecular Field Analysis [14'151 (CoMFA) may be used to investigate the steric and electrostatic fields which define templates. However, CoMFA requires that target molecules be aligned identically before any comparison ac be made, and therefore analyses are restricted only to very closely related sets of templating agents. It may however provide a tool which can identify the template charge characteristics necessary to produce zeolites with a certain Si / A1 ratio, or crystal morphology. It has been suggested that CoMFA be applied directly to describe the quantitative structure-activity relationships of zeolites with known catalytic properties, t141
4. CONCLUSIONS The principal axes of inertia provide a basic quantitative description of the shape of organic templating agents, and if taken into account with molecular volumes, can increase the current understanding of the influence of template structure on the structure of zeolite pore systems. These descriptors also offer an indication as to the differing manners by which various templates are incorporated into the channel system of any one zeolite, t16] The use of structure descriptors is complementary to Monte Carlo-Simulated Annealing [4'171 and, perhaps, D e N o v o methods, and could establish which shape criteria are necessary to allow rational computer assisted design of novel catalytic zeolite systems to be realised. New and improved templating agents could be sought by screening the RN values of candidate molecules, and comparing them against the known values from previously successful templates. Novel templates may also be pursued using the concept of shape and sub-shape, whereby, for instance, cheaper and smaller agents may be identified as effective sub-units of larger molecules, and vice versa. Plotting a new template upon a principal axes of inertia 'shape-space diagram' may predict which zeolite structures it might produce in addition to any
124 (more desirable) target type, and then the optimum reaction conditions necessary for its deployment may begin to be established. The authors are currently seeking to supplement the described preliminary survey by accounting for the importance of template shape in respect to other variables in zeolite synthesis, notably the electrostatic profiles of organic templates, gel chemistry and reaction conditions. A detailed chemometric or quantitative structure-property relationships (QSPR) analysis of the available data may yield information which will permit specific preparative methods to be identified for the deployment of novel templating agents.
REFERENCES
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H. Gies and B. Marler, Zeolites, 12, (1992), 42. B.M. Lok, T.R. Cannan, and C.A. Messina, Zeolites, 3, (1983), 282. L.B. McCusker, Materials Science Forum, 133-136, (1993), 423. A.P. Stevens, A.M. Gorman, C.M. Freeman, and P.A. Cox, submitted to J. Chem. Soc., Faraday Trans., (November 1995). A.P. Stevens and P.A. Cox, J. Chem. Soc., Chem. Commun., (1995), 343. See, for example, "Modelling of Structure and Reactivity in Zeolites," (ed. C.R.A. Catlow), Academic Press, London, (1992). C.M. Freeman, D.W. Lewis, T.V. Harris, A.K. Cheetham, N.J. Henson, P.A. Cox, A.M. Gorman, S.M. Levine, J.M. Newsam, E. Hemandez, and C.R.A. Catlow, in "ComputerAided Molecular Design: Agrochemicals, Materials and Pharmaceuticals," (eds. C.H. Reynolds, M.K. Holloway, and H.K. Cox), ACS, Washington DC, (1995), 326. D. Livingstone, "Data Analysis for Chemists," Oxford University Press, (1995). P.M. Dean, "Molecular Foundations of Drug-Receptor Interaction," Cambridge University Press, (1987), 254. P.C. Jurs, S.L. Dixon, and L.M. Egolf, in "Chemometric Methods' in Molecular Design, Methods' and Principles in Medicinal Chemistry Volume 2," (ed. H. van de Waterbeemd), VCH, (1995), 15. A list of calculated values for the principal axes of inertia relating to the templates included in this study are available on request from the authors, together with full details of the literature source for each. Discover 3.1 program, Insight H User Guide, Version 2.3.6, Biosym Technologies Inc., San Diego, USA, (1993). Tsar version 2.02, Oxford Molecular Ltd., UK, (1993). R.D. Cramer IU, D.E. Patterson, and J.D. Bunce, J. Am. Chem. Soc., 110, (1988), 5959. K.H. Kim, in "Molecular Similarity in Drug Design," (ed. P.M. Dean), Blackie Academic and Professional, Glasgow, (1995), 291. See also R.E. Boyett, A.P. Stevens, M.G. Ford, and P.A. Cox, to be submitted to Zeolites, 1996. P.A. Cox, A.P. Stevens, L. Banting, and A.M. Gorman, in "Zeolites and Related Microporous Materials: State of the Art 1994," Studies in Surface Science and Catalysis, Vol 84, (eds. J. Weitkamp, H.G. Karge, H. Pfeifer, and W. H/51derich), Elsevier, Science B.V., (1994), 2115.