- Email: [email protected]
Growth models in microporous materials
Microporous and Mesoporous Materials 48 (2001) 1±9
www.elsevier.com/locate/micromeso
Growth models in microporous materials Michael W. Anderson a,*, Jonathan R. Agger a, Noreen Hanif a, Osamu Terasaki b a
Department of Chemistry, UMIST Centre for Microporous Materials, UMIST, P.O. Box 88, Manchester M60 1QD, UK b Department of Physics, CIR and CREST(JST), Tohoku University, Aramaki Aoba, Sendai 980-8578, Japan Received 6 August 2000; received in revised form 19 March 2001; accepted 19 March 2001
Abstract Crystallisation pathways in framework materials, such as zeolites, have been monitored using a combination of atomic force microscopy, high-resolution electron microscopy and modelling. The principle conclusions of this study is that many open-framework crystals grow via a layer growth mechanism. The layers have a thickness which is related to the unit cell parameter in the direction of growth. The development of each layer can be sub-divided into a number of individual growth processes including nucleation of a new layer (or terrace), growth at edge sites and kink sites. Modelling of atomic force micrographs yields relative probabilities for these individual growth processes and reveals that growth at kink sites is favoured with nucleation of new terraces the slowest process. The layer mechanisms proposed explain the incorporation of defects in framework structures and further elucidate the role of structure directing agents. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Zeolite; Crystal growth; Atomic force microscopy; High-resolution electron microscopy
1. Introduction Catalysis and defects are two closely related phenomena. Most heterogeneous catalytic processes occur on apparently ``well-de®ned periodic'' solids ± by well-de®ned periodic solids we mean either that the solid possesses three-dimensional order as in a crystal, or two-dimensional order as in a facet of a crystal. However, it is clear that these well-de®ned solids are only the background to the details which govern their catalytic properties. Catalysis is generally a property which occurs
* Corresponding author. Tel.: +44-161-200-4465; fax: +44161-236-7677. E-mail addresses: [email protected] (M.W. Anderson), [email protected] (O. Terasaki).
at a defect site within a solid ± by defect we mean any part of the overall structure which does not necessarily occur in a regular pattern ± but we should be clear that defects are generally very ``well-de®ned locally'' in nature. This well-de®ned local nature of a defect site is a consequence of the well-de®ned periodic background in which it is housed. In the context of catalysis a defect can either be an inherent property of a solid or it can be created during the catalytic event ± as for instance an activated transition state. In special circumstances the concentration of these defect sites becomes so high that they will arrange themselves in a periodic array and in this limit (but only in the limit) could they be considered no longer to be defects. Consequently, understanding heterogeneous catalysts and catalysis requires an understanding of both the well-de®ned periodic nature
1387-1811/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 1 ) 0 0 3 2 4 - 9
2
M.W. Anderson et al. / Microporous and Mesoporous Materials 48 (2001) 1±9
of solids but also, and more importantly, the wellde®ned local nature of solids. In order to understand the detailed nature of framework solids it is very important to understand the crystallisation pathway. Defects in framework materials are incorporated as a consequence of the crystal growth mechanism. In this paper we use a combination of atomic force microscopy (AFM), high-resolution electron microscopy (HREM) and modelling to elucidate the details of the mechanisms of crystal growth in a number of framework materials. The mechanisms proposed, which are generally a layer type mechanism are responsible for all the dierent defect structures observed in these framework materials. Further, the role of structure directing agents (templates) can be considered in terms of these growth mechanisms and strategies for their use are discussed. 2. Experimental AFM images were recorded on a digital instruments nanoscope multimode microscope operating in tapping mode. Sample preparation involved embedding the microcrystallites in a thermoplastic ®xative on a metal stub. Using an optical microscope mounted on the AFM, it is relatively easy to land the tip on crystal faces larger than 10 10 lm2 . For smaller crystallites it is usually necessary to land the tip on the thermoplastic close to a crystallite and scan the local area to ®nd the exact location of the crystal face. A ®rst order plane®t was conducted on the images in the x and y directions to level the crystal terraces. Beyond the crystal edges, images contain information on the tip shape only and do not contain topographical information. High-resolution transmission electron micrographs and electron diraction patterns were recorded on a JEM-4000EX, JEOL microscope operating at 400 kV. Specimens were dispersed in acetone by a ultrasonic washer and deposited on a microgrid. Modelling of AFM images was performed by a program written in Mathematica which assigns probabilities to the individual nucleation phenomena described in the text. Growth is then
performed randomly at these sites in accord with these given probabilities. 3. Understanding crystal growth (role of defects) Thermodynamically all crystals contain defects. In dense phase structures of highly ionic species where ions are packed through coulombic interaction then the types of defects present are well understood and fairly limited. However, for covalently bonded systems, and in particular for open framework structures, the types of defects are as diverse as the number of dierent structures that can be formed. The types of defects that are incorporated into a framework type structure are often a consequence of the crystal growth process ± or from the opposite point of view the crystal growth process is often severely modi®ed by the incorporation of defects. Some defects can be studied very successfully by HREM techniques, and this is discussed in a later section. However, some defects can only be inferred via a combination of circumstantial evidence from techniques such as NMR, IR and diraction. In this section we show how AFM [1±3] can be used to glean information about how zeolite crystals grow and lessons for the incorporation of defect structures. Three systems are considered, zeolite Y, zeolite A and silicalite, and the AFM images of these are shown in Figs. 1±3 respectively. The zeolite Y crystals have essentially an octahedral habit with the (1 1 1) surfaces being predominantly exposed. AFM images reveal that the surface of the crystal is composed of terraces with (i) height 15 A, equivalent to one faujasite sheet (ii) triangular shape consistent with the threefold [1 1 1] axis and (iii) the terraces get closer together towards the edge of the crystal. From this last point it can be shown that the area of the terraces is growing at a constant rate, consistent with a terrace-ledge-kink mechanism. A similar situation is observed on the (1 0 0) face of zeolite A where again terraces are consistent observed this time with height 12 A with the height of a sodalite cage and double four ring unit. (Zeolite A has also been studied using AFM by Sugiyama et al. under dierent synthetic conditions [4].) Two fundamental dierences are
M.W. Anderson et al. / Microporous and Mesoporous Materials 48 (2001) 1±9
3
observed, ®rst, the number of surface nuclei is much higher than in zeolite Y, second where the growing terraces merge a curved terrace edge is observed. From a detailed theoretical modelling of these surface features it can be further established that the faces are growing via a terrace-ledge-kink mechanism and further that the relative growth rate for the surfaces is: 500 surface nucleation rate 15 terrace edge nucleation rate kink nucleation rate
Fig. 1. Atomic force micrograph of the (1 1 1) surface of zeolite Y.
Fig. 2. Atomic force micrograph of the (1 0 0) surface of zeolite A and the de®nition of growth sites.
Fig. 3. Atomic force micrograph of the (0 1 0) surface of silicalite grown at (a) 180°C and (b) 130°C.
See Fig. 2 for a description of the dierent site types. Fig. 4 shows a series of calculated images based upon this theoretical study. All the main features observed in the experimental AFM images are reproduced in the simulation. These include: the correct surface density of terraces; the square nature of the individual terraces; the curved growth fronts between coalescing terraces. Although there will be some error associated with these dierent nucleation rates, this is the ®rst study that reveals the relative rates of these individual growth processes. It is likely that similar theoretical modelling of other zeolite systems will yield individual nucleation rates. The next step is to determine how these relative rates can be in¯uenced through the modi®cation of synthetic conditions and we now have the tools available to monitor these processes. In zeolite A when two terraces merge, owing to the alternation of Si and Al in the underlying structure and the necessary registry of the layer above, no defects are created. However, in zeolite Y if multiple nucleation occurs on a growing surface then it can occur in one of two possible con®gurations. When terraces of dierent con®guration merge they do not join and a line defect is created along the boundary [5]. Fig. 5 shows the HREM along the surface of a zeolite Y crystallite which both reveals the surface terraces (seen as steps in projection) and also the details of the surface termination. The last half pores at the crystal surface are clearly apparent and the nature of the surface termination can be ascertained [6]. The consequences of multiple nucleation can be seen in the HREM micrograph in Fig. 6 where a
4
M.W. Anderson et al. / Microporous and Mesoporous Materials 48 (2001) 1±9
Fig. 5. HREM of zeolite Y showing the surface structure.
Fig. 6. HREM of a vanishing twin boundary in zeolite Y.
Fig. 4. Simulated growth features in zeolite A showing progressive steps in the calculation.
vanishing twin boundary is observed. Such a defect is healed after the growth of a number of layers and the mechanism for this process is shown schematically in Fig. 7. In order to prevent such defects occurring it would be necessary to decrease the rate of surface nucleation with respect to edge and kink site nucleation, however, generally speaking this is not a major problem in the case of
zeolite Y. The example given later of ETS-10, however, shows a similar phenomenon which is much more extensive. In silicalite the situation regarding defect incorporation is even more severe. Fig. 3 shows the AFM images recorded on the (0 1 0) faces of two dierent crystals, one prepared at 130°C and one at 180°C. On the low temperature preparation are observed with surface terraces of height 10 A no preferential growth direction for the terrace edges. The height corresponds to a double-®vering chain. In the high temperature preparation with very high terraces are observed (over 100 A)
M.W. Anderson et al. / Microporous and Mesoporous Materials 48 (2001) 1±9
5
Fig. 8. Schematic diagram of the growth of silicalite and the incorporation of linear defects. The bars delineate pentasil chains which must be connected left-haded to right-handed. The perfect structure has these related by inversion symmetry, indicated by black dots and mirror symmetry indicated by black lines. The thick white lines indicate where hydroxyl clusters will be incorporated in long chains along the defect.
Fig. 7. Explanation of the vanishing twin boundary in terms of a layer growth in zeolite Y with a double nucleation on a growing surface.
preferential growth parallel to the edges of the crystallites. These very high terraces have essentially been pinned by ordered defects nucleating on the growing surface. Fig. 8 shows a schematic diagram of this eect whereby a double-®ve-ring chain nucleates with the wrong orientation. The defect can only propagate parallel to the crystal edges thereby pinning the growing terraces also parallel to the crystal edge. Similar defects have been implicated for features in the electron diffraction and HREM of MEL structure [7]. The consequence is that the growing terrace collides with the defect and is unable to bond. A whole chain of hydroxyl clusters will be generated along the entire length of the crystal (which may be available for functionalisation and thereby catalytic activity). The growth of the layer is substan-
tially slowed and a ``cli face'' is developed along the defect. The growth will eventually overcome the defect, however, chains of hydroxyl groups will remain within the crystal. The presence of high concentrations of such defects in silicalite is possibly responsible for the appearance of a bow-tie eect in the optical micrographs of these crystals. It appears that in the low temperature synthesis the presence of such defects is very low.
4. Controlling intergrowths via templating (lessons from AFM and HREM) The lessons learnt from the AFM and HREM studies, in particular for zeolite Y, demonstrate that the crystals grow by a layer type growth mechanism of faujasite sheets. This also helps to understand intergrowths of related cubic and hexagonal zeolite Y materials known as FAU and EMT respectively. FAU has an ABCABC stacking of pores which is a result of adjacent faujasite sheets linked by an inversion centre. EMT has an ABABAB stacking of pores which is a result of adjacent faujasite sheets linked by mirror symmetry (see HREM in Fig. 9). The latter
6
M.W. Anderson et al. / Microporous and Mesoporous Materials 48 (2001) 1±9
Fig. 10. (1 1 1) surface of zeolite Y showing surface decoration by 18-crown-6 which is subsequently clathrated by the growth of a subsequent layer.
Fig. 9. HREM of (a) zeolite Y cubic phase (FAU), (b) zeolite Y hexagonal phase (EMAT) and (c) an intergrowth structure synthesised with a mixture of 18-crown-6 and 15-crown-5 templates.
structure can only be synthesised in its pure form by the use of the structure-directing agent 18crown-6. The 18-crown-6 decorates the growing (1 1 1) faujasite surface by locking into half-pockets (see Fig. 10). The presence of the crown-ether forces the next layer to grow in a mirror relation thereby clathrating the 18-crown-6 within the structure. In the presence of mixtures of crown ethers (18-crown-6 directing EMT and 15-crown-5 directing FAU) intergrowths are observed (see Fig. 9) [6]. But the occurrence of EMT and FAU is not random and an oscillation between the two polymorphs is observed [8]. This is viewed as a transport limited diusion of structure-directing agent to the growing surface. In the presence of a high enough concentration of 18-crown-6 EMT
will form. But the formation of EMT consumes 18crown-6 from the supernatent solution in the vicinity of the growing surface. Below a threshold concentration FAU structure is preferred until the concentration of 18-crown-6 builds up. Such a mechanism is further substantiated by syntheses in stirred growth media whereby the concentration of structure-directing agents is kept constant. In this situation at a concentration of 30% 18-crown-6 the product immediately switches from FAU to EMT ± this establishes the threshold concentration of 18crown-6 required for EMT formation (see Fig. 11). Recently there has been a great deal of interest in synthesising novel microporous materials with mixed coordination of metals in the framework. In particular the mixed tetrahedral/octahedral materials whereby the tetrahedral element may be Si, Al, Ga etc. and the octahedral element is Ti, V, Sn, Zr etc. The archetypal material in this class is ETS10 (Engelhard titanosilicate-10) ®rst synthesised in 1989 [9] but the structure only revealed in 1994 [10±12]. This material is interesting for a number of features: (i) it has a three-dimensional 12-ring pore system giving good diusional access; (ii) it has a very high cation exchange capacity, equiva-
M.W. Anderson et al. / Microporous and Mesoporous Materials 48 (2001) 1±9
Fig. 11. Graph showing the proportion of EMT structure present in zeolite Y samples synthesised with a mixture of 18crown-6 and 15-crown-5. The graph compares a static synthesis and a stirred synthesis. The proportion of each phase was determined by HREM and X-ray diraction measurments.
lent to zeolite X, however, the framework charge sites are divalent TiO26 ; (iii) the structure is inherently an intergrowth structure consisting of randomly stacked layers, if this layer stacking could be controlled a chiral polymorph with a spiral channel could be synthesised; (iv) the material inherently has a high concentration of defects which both increase the internal dimensions of the voids and potentially provide sites for catalytic activity; (v) the material is particularly active and selective for base catalysis; (vi) ETS-10 is very thermally and hydrothermally stable. Consequently, ETS-10 and related materials are potentially a very exciting new class of microporous catalyst. In ETS-10 the intergrowth structure and
7
defects are a consequence again of the layer-bylayer growth mechanism (see Fig. 12). New layers nucleate in one of four possible positions, two of these can be seen in the electron micrograph and results in a stacking of the large pores to the left or right (further stacking in and out of the plane of the paper cannot be seen in the micrograph). The intergrowth structure is then formed as a random stacking of layers to the left or right as you proceed vertically up the micrograph. If at one layer there is double nucleation with one nuclei to the left and one to the right then when these nuclei propagate into a layer and meet they will form a large pore defect at the junction. Many such defects can be seen in the micrograph and are a constant feature in ETS-10. With this knowledge of the crystal growth of ETS-10 we can develop strategies to control both the intergrowth structure and the defect density. First, controlling the intergrowth structure. ETS10 is a particularly interesting structure because, similar to zeolite beta, one of the ordered intergrowths, with an alternating stacking of channels to the left then right gives a 12-ring spiral channel in the third direction. Consequently in order to synthesise this polymorph it will be necessary to control the surface nucleation, most probably via templating. The strategy to adopt will be to consider the growing surface topology then design a template that will both lock in to this surface then direct the next layer to grow to the left then right producing a spiral. A chiral molecule will be required but long spiral molecules are inappropriate as they will only ®ll the spiral channel and not lock into the growing surface. Second, to control the defect density. There are reasons why it is desirable to both increase and decrease the density of the open pore defects. Increasing the density will create a material with many ultra-large pores, decreasing the density may improve the stability of the material. The key is in increasing or decreasing the rate of nucleation at the surface relative to the propagation of a new layer across the surface. Increasing the nucleation rate will increase the density of defect sites and vice versa. Strategies to be adopted may include altering the temperature or supersaturation in the synthesis gel, adding surface binding agents to
8
M.W. Anderson et al. / Microporous and Mesoporous Materials 48 (2001) 1±9
Fig. 12. HREM of ETS-10 showing the layer growth which results in both the defects, which are the double pores, and the intergrowth structure seen as a random stacking of layers.
slow down surface nucleation, changing the source of nutrient etc. Clearly, there is still a long way to go to be able to control the individual growth processes within one crystal system, however, again knowledge of the mechanism of the growth processes allows the development of suitable synthesis strategies.
5. Growth units It is important to stress in this work that our microscopy studies do not reveal information about individual growth units. It will be necessary to perform associated spectroscopic studies to glean information in this area. However, our work does indicate that the growth process is not continuous, i.e. there are certain closed structures which are preferable and the growth is a stepwise mechanism. In this manner we are able to break down the process ®rst into layers, then sub-divide the layers into growth at edge sites and kink sites. The nature of the closed structures is unclear and theoretical studies such as those performed on calcite by Leeuw and Parker will be necessary [13].
However, we may speculate that the closed structures are probably complete cages such as sodalite cages or double six-rings etc. It should also be stressed that the terms edge and kink sites should be considered in a somewhat loose sense and not in the strict de®nition normally associated with crystal growth in dense phases. In a framework structure surface, edge and kink sites will be sites with increasing numbers of attachment points for the next closed structure.
6. Conclusions In conclusion we have shown that generally many framework structures grow via a layer growth process. Templating occurs by a surface decoration whereby the organic structure-directing agents locks into the growing face and keys the growth of the subsequent layer. Defects and intergrowths on framework structures are a consequence of the speci®c growth process and in principle these could be controlled by modifying the rates of individual growth processes. AFM as a
M.W. Anderson et al. / Microporous and Mesoporous Materials 48 (2001) 1±9
tool is able to reveal the relative rates of individual growth processes. Acknowledgements We would like to thank EPSRC (especially Advanced Fellowship AF/990985 to JRA), British Council, Daiwa Award and CREST (JST) for funding.
References [1] M.W. Anderson, J.R. Agger, J.T. Thornton, N. Forsyth, Angew. Chem. Int. Ed. Engl. 38 (1996) 1210. [2] J.R. Agger, N. Pervaiz, A.K. Cheetham, M.W. Anderson, J. Am. Chem. Soc. 120 (1998) 10754. [3] M.W. Anderson, J.R. Agger, N. Pervaiz, S.J. Weigel, A.K. Cheetham, in: Treacey, et al. (Eds.), Proc. 12th Int. Zeolite Conf., Baltimore, July 1998, MRS, 1487.
9
[4] S. Sugiyama, S. Yamamoto, O. Matsuoka, H. Nozoye, J. Yu, Z. Gaugshang, S. Qiu, O. Terasaki, Micorporous Mesoporous Mater. 1 (1999) 28. [5] T. Ohsuna, O. Terasaki, V. Alfredsson, J.-O. Bovin, D. Watanabe, S.W. Carr, M.W. Anderson, Proc. R. Soc. Lond. A 452 (1996) 715. [6] V. Alfredsson, T. Ohsuna, O. Terasaki, J.-O. Bovin, Angew. Chem. Int. Ed. Engl. 32 (1993) 1210. [7] T. Ohsuna, O. Terasaki, Y. Nakagawa, S.I. Zones, K. Hiraga, Phys. Chem. B 101 (1997) 9881. [8] O. Terasaki, T. Ohsuna, V. Alfredsson, J.-O. Bovin, D. Watanabe, S.W. Carr, M.W. Anderson, Chem. Mater. 5 (1993) 452. [9] S.M. Kuznicki, US Patent 4853202 (1989). [10] M.W. Anderson, O. Terasaki, T. Ohsuna, A. Philippou, S.P. MacKay, A. Ferreira, J. Rocha, S. Lidin, Nature 367 (1994) 347. [11] M.W. Anderson, O. Terasaki, T. Ohsuna, P.J. O'Malley, A. Philippou, S.P. MacKay, A. Ferreira, J. Rocha, S. Lidin, Philos. Mag. B 71 (1995) 813. [12] T. Ohsuna, O. Terasaki, D. Watanabe, M.W. Anderson, S. Lidin, Stud. Surf. Sci. Catal. 84 (1994) 413. [13] N.H. de Leeuw, S.C. Parker, J. Phys. Chem. B 102 (1998) 2914.
www.elsevier.com/locate/micromeso
Growth models in microporous materials Michael W. Anderson a,*, Jonathan R. Agger a, Noreen Hanif a, Osamu Terasaki b a
Department of Chemistry, UMIST Centre for Microporous Materials, UMIST, P.O. Box 88, Manchester M60 1QD, UK b Department of Physics, CIR and CREST(JST), Tohoku University, Aramaki Aoba, Sendai 980-8578, Japan Received 6 August 2000; received in revised form 19 March 2001; accepted 19 March 2001
Abstract Crystallisation pathways in framework materials, such as zeolites, have been monitored using a combination of atomic force microscopy, high-resolution electron microscopy and modelling. The principle conclusions of this study is that many open-framework crystals grow via a layer growth mechanism. The layers have a thickness which is related to the unit cell parameter in the direction of growth. The development of each layer can be sub-divided into a number of individual growth processes including nucleation of a new layer (or terrace), growth at edge sites and kink sites. Modelling of atomic force micrographs yields relative probabilities for these individual growth processes and reveals that growth at kink sites is favoured with nucleation of new terraces the slowest process. The layer mechanisms proposed explain the incorporation of defects in framework structures and further elucidate the role of structure directing agents. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Zeolite; Crystal growth; Atomic force microscopy; High-resolution electron microscopy
1. Introduction Catalysis and defects are two closely related phenomena. Most heterogeneous catalytic processes occur on apparently ``well-de®ned periodic'' solids ± by well-de®ned periodic solids we mean either that the solid possesses three-dimensional order as in a crystal, or two-dimensional order as in a facet of a crystal. However, it is clear that these well-de®ned solids are only the background to the details which govern their catalytic properties. Catalysis is generally a property which occurs
* Corresponding author. Tel.: +44-161-200-4465; fax: +44161-236-7677. E-mail addresses: [email protected] (M.W. Anderson), [email protected] (O. Terasaki).
at a defect site within a solid ± by defect we mean any part of the overall structure which does not necessarily occur in a regular pattern ± but we should be clear that defects are generally very ``well-de®ned locally'' in nature. This well-de®ned local nature of a defect site is a consequence of the well-de®ned periodic background in which it is housed. In the context of catalysis a defect can either be an inherent property of a solid or it can be created during the catalytic event ± as for instance an activated transition state. In special circumstances the concentration of these defect sites becomes so high that they will arrange themselves in a periodic array and in this limit (but only in the limit) could they be considered no longer to be defects. Consequently, understanding heterogeneous catalysts and catalysis requires an understanding of both the well-de®ned periodic nature
1387-1811/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 1 ) 0 0 3 2 4 - 9
2
M.W. Anderson et al. / Microporous and Mesoporous Materials 48 (2001) 1±9
of solids but also, and more importantly, the wellde®ned local nature of solids. In order to understand the detailed nature of framework solids it is very important to understand the crystallisation pathway. Defects in framework materials are incorporated as a consequence of the crystal growth mechanism. In this paper we use a combination of atomic force microscopy (AFM), high-resolution electron microscopy (HREM) and modelling to elucidate the details of the mechanisms of crystal growth in a number of framework materials. The mechanisms proposed, which are generally a layer type mechanism are responsible for all the dierent defect structures observed in these framework materials. Further, the role of structure directing agents (templates) can be considered in terms of these growth mechanisms and strategies for their use are discussed. 2. Experimental AFM images were recorded on a digital instruments nanoscope multimode microscope operating in tapping mode. Sample preparation involved embedding the microcrystallites in a thermoplastic ®xative on a metal stub. Using an optical microscope mounted on the AFM, it is relatively easy to land the tip on crystal faces larger than 10 10 lm2 . For smaller crystallites it is usually necessary to land the tip on the thermoplastic close to a crystallite and scan the local area to ®nd the exact location of the crystal face. A ®rst order plane®t was conducted on the images in the x and y directions to level the crystal terraces. Beyond the crystal edges, images contain information on the tip shape only and do not contain topographical information. High-resolution transmission electron micrographs and electron diraction patterns were recorded on a JEM-4000EX, JEOL microscope operating at 400 kV. Specimens were dispersed in acetone by a ultrasonic washer and deposited on a microgrid. Modelling of AFM images was performed by a program written in Mathematica which assigns probabilities to the individual nucleation phenomena described in the text. Growth is then
performed randomly at these sites in accord with these given probabilities. 3. Understanding crystal growth (role of defects) Thermodynamically all crystals contain defects. In dense phase structures of highly ionic species where ions are packed through coulombic interaction then the types of defects present are well understood and fairly limited. However, for covalently bonded systems, and in particular for open framework structures, the types of defects are as diverse as the number of dierent structures that can be formed. The types of defects that are incorporated into a framework type structure are often a consequence of the crystal growth process ± or from the opposite point of view the crystal growth process is often severely modi®ed by the incorporation of defects. Some defects can be studied very successfully by HREM techniques, and this is discussed in a later section. However, some defects can only be inferred via a combination of circumstantial evidence from techniques such as NMR, IR and diraction. In this section we show how AFM [1±3] can be used to glean information about how zeolite crystals grow and lessons for the incorporation of defect structures. Three systems are considered, zeolite Y, zeolite A and silicalite, and the AFM images of these are shown in Figs. 1±3 respectively. The zeolite Y crystals have essentially an octahedral habit with the (1 1 1) surfaces being predominantly exposed. AFM images reveal that the surface of the crystal is composed of terraces with (i) height 15 A, equivalent to one faujasite sheet (ii) triangular shape consistent with the threefold [1 1 1] axis and (iii) the terraces get closer together towards the edge of the crystal. From this last point it can be shown that the area of the terraces is growing at a constant rate, consistent with a terrace-ledge-kink mechanism. A similar situation is observed on the (1 0 0) face of zeolite A where again terraces are consistent observed this time with height 12 A with the height of a sodalite cage and double four ring unit. (Zeolite A has also been studied using AFM by Sugiyama et al. under dierent synthetic conditions [4].) Two fundamental dierences are
M.W. Anderson et al. / Microporous and Mesoporous Materials 48 (2001) 1±9
3
observed, ®rst, the number of surface nuclei is much higher than in zeolite Y, second where the growing terraces merge a curved terrace edge is observed. From a detailed theoretical modelling of these surface features it can be further established that the faces are growing via a terrace-ledge-kink mechanism and further that the relative growth rate for the surfaces is: 500 surface nucleation rate 15 terrace edge nucleation rate kink nucleation rate
Fig. 1. Atomic force micrograph of the (1 1 1) surface of zeolite Y.
Fig. 2. Atomic force micrograph of the (1 0 0) surface of zeolite A and the de®nition of growth sites.
Fig. 3. Atomic force micrograph of the (0 1 0) surface of silicalite grown at (a) 180°C and (b) 130°C.
See Fig. 2 for a description of the dierent site types. Fig. 4 shows a series of calculated images based upon this theoretical study. All the main features observed in the experimental AFM images are reproduced in the simulation. These include: the correct surface density of terraces; the square nature of the individual terraces; the curved growth fronts between coalescing terraces. Although there will be some error associated with these dierent nucleation rates, this is the ®rst study that reveals the relative rates of these individual growth processes. It is likely that similar theoretical modelling of other zeolite systems will yield individual nucleation rates. The next step is to determine how these relative rates can be in¯uenced through the modi®cation of synthetic conditions and we now have the tools available to monitor these processes. In zeolite A when two terraces merge, owing to the alternation of Si and Al in the underlying structure and the necessary registry of the layer above, no defects are created. However, in zeolite Y if multiple nucleation occurs on a growing surface then it can occur in one of two possible con®gurations. When terraces of dierent con®guration merge they do not join and a line defect is created along the boundary [5]. Fig. 5 shows the HREM along the surface of a zeolite Y crystallite which both reveals the surface terraces (seen as steps in projection) and also the details of the surface termination. The last half pores at the crystal surface are clearly apparent and the nature of the surface termination can be ascertained [6]. The consequences of multiple nucleation can be seen in the HREM micrograph in Fig. 6 where a
4
M.W. Anderson et al. / Microporous and Mesoporous Materials 48 (2001) 1±9
Fig. 5. HREM of zeolite Y showing the surface structure.
Fig. 6. HREM of a vanishing twin boundary in zeolite Y.
Fig. 4. Simulated growth features in zeolite A showing progressive steps in the calculation.
vanishing twin boundary is observed. Such a defect is healed after the growth of a number of layers and the mechanism for this process is shown schematically in Fig. 7. In order to prevent such defects occurring it would be necessary to decrease the rate of surface nucleation with respect to edge and kink site nucleation, however, generally speaking this is not a major problem in the case of
zeolite Y. The example given later of ETS-10, however, shows a similar phenomenon which is much more extensive. In silicalite the situation regarding defect incorporation is even more severe. Fig. 3 shows the AFM images recorded on the (0 1 0) faces of two dierent crystals, one prepared at 130°C and one at 180°C. On the low temperature preparation are observed with surface terraces of height 10 A no preferential growth direction for the terrace edges. The height corresponds to a double-®vering chain. In the high temperature preparation with very high terraces are observed (over 100 A)
M.W. Anderson et al. / Microporous and Mesoporous Materials 48 (2001) 1±9
5
Fig. 8. Schematic diagram of the growth of silicalite and the incorporation of linear defects. The bars delineate pentasil chains which must be connected left-haded to right-handed. The perfect structure has these related by inversion symmetry, indicated by black dots and mirror symmetry indicated by black lines. The thick white lines indicate where hydroxyl clusters will be incorporated in long chains along the defect.
Fig. 7. Explanation of the vanishing twin boundary in terms of a layer growth in zeolite Y with a double nucleation on a growing surface.
preferential growth parallel to the edges of the crystallites. These very high terraces have essentially been pinned by ordered defects nucleating on the growing surface. Fig. 8 shows a schematic diagram of this eect whereby a double-®ve-ring chain nucleates with the wrong orientation. The defect can only propagate parallel to the crystal edges thereby pinning the growing terraces also parallel to the crystal edge. Similar defects have been implicated for features in the electron diffraction and HREM of MEL structure [7]. The consequence is that the growing terrace collides with the defect and is unable to bond. A whole chain of hydroxyl clusters will be generated along the entire length of the crystal (which may be available for functionalisation and thereby catalytic activity). The growth of the layer is substan-
tially slowed and a ``cli face'' is developed along the defect. The growth will eventually overcome the defect, however, chains of hydroxyl groups will remain within the crystal. The presence of high concentrations of such defects in silicalite is possibly responsible for the appearance of a bow-tie eect in the optical micrographs of these crystals. It appears that in the low temperature synthesis the presence of such defects is very low.
4. Controlling intergrowths via templating (lessons from AFM and HREM) The lessons learnt from the AFM and HREM studies, in particular for zeolite Y, demonstrate that the crystals grow by a layer type growth mechanism of faujasite sheets. This also helps to understand intergrowths of related cubic and hexagonal zeolite Y materials known as FAU and EMT respectively. FAU has an ABCABC stacking of pores which is a result of adjacent faujasite sheets linked by an inversion centre. EMT has an ABABAB stacking of pores which is a result of adjacent faujasite sheets linked by mirror symmetry (see HREM in Fig. 9). The latter
6
M.W. Anderson et al. / Microporous and Mesoporous Materials 48 (2001) 1±9
Fig. 10. (1 1 1) surface of zeolite Y showing surface decoration by 18-crown-6 which is subsequently clathrated by the growth of a subsequent layer.
Fig. 9. HREM of (a) zeolite Y cubic phase (FAU), (b) zeolite Y hexagonal phase (EMAT) and (c) an intergrowth structure synthesised with a mixture of 18-crown-6 and 15-crown-5 templates.
structure can only be synthesised in its pure form by the use of the structure-directing agent 18crown-6. The 18-crown-6 decorates the growing (1 1 1) faujasite surface by locking into half-pockets (see Fig. 10). The presence of the crown-ether forces the next layer to grow in a mirror relation thereby clathrating the 18-crown-6 within the structure. In the presence of mixtures of crown ethers (18-crown-6 directing EMT and 15-crown-5 directing FAU) intergrowths are observed (see Fig. 9) [6]. But the occurrence of EMT and FAU is not random and an oscillation between the two polymorphs is observed [8]. This is viewed as a transport limited diusion of structure-directing agent to the growing surface. In the presence of a high enough concentration of 18-crown-6 EMT
will form. But the formation of EMT consumes 18crown-6 from the supernatent solution in the vicinity of the growing surface. Below a threshold concentration FAU structure is preferred until the concentration of 18-crown-6 builds up. Such a mechanism is further substantiated by syntheses in stirred growth media whereby the concentration of structure-directing agents is kept constant. In this situation at a concentration of 30% 18-crown-6 the product immediately switches from FAU to EMT ± this establishes the threshold concentration of 18crown-6 required for EMT formation (see Fig. 11). Recently there has been a great deal of interest in synthesising novel microporous materials with mixed coordination of metals in the framework. In particular the mixed tetrahedral/octahedral materials whereby the tetrahedral element may be Si, Al, Ga etc. and the octahedral element is Ti, V, Sn, Zr etc. The archetypal material in this class is ETS10 (Engelhard titanosilicate-10) ®rst synthesised in 1989 [9] but the structure only revealed in 1994 [10±12]. This material is interesting for a number of features: (i) it has a three-dimensional 12-ring pore system giving good diusional access; (ii) it has a very high cation exchange capacity, equiva-
M.W. Anderson et al. / Microporous and Mesoporous Materials 48 (2001) 1±9
Fig. 11. Graph showing the proportion of EMT structure present in zeolite Y samples synthesised with a mixture of 18crown-6 and 15-crown-5. The graph compares a static synthesis and a stirred synthesis. The proportion of each phase was determined by HREM and X-ray diraction measurments.
lent to zeolite X, however, the framework charge sites are divalent TiO26 ; (iii) the structure is inherently an intergrowth structure consisting of randomly stacked layers, if this layer stacking could be controlled a chiral polymorph with a spiral channel could be synthesised; (iv) the material inherently has a high concentration of defects which both increase the internal dimensions of the voids and potentially provide sites for catalytic activity; (v) the material is particularly active and selective for base catalysis; (vi) ETS-10 is very thermally and hydrothermally stable. Consequently, ETS-10 and related materials are potentially a very exciting new class of microporous catalyst. In ETS-10 the intergrowth structure and
7
defects are a consequence again of the layer-bylayer growth mechanism (see Fig. 12). New layers nucleate in one of four possible positions, two of these can be seen in the electron micrograph and results in a stacking of the large pores to the left or right (further stacking in and out of the plane of the paper cannot be seen in the micrograph). The intergrowth structure is then formed as a random stacking of layers to the left or right as you proceed vertically up the micrograph. If at one layer there is double nucleation with one nuclei to the left and one to the right then when these nuclei propagate into a layer and meet they will form a large pore defect at the junction. Many such defects can be seen in the micrograph and are a constant feature in ETS-10. With this knowledge of the crystal growth of ETS-10 we can develop strategies to control both the intergrowth structure and the defect density. First, controlling the intergrowth structure. ETS10 is a particularly interesting structure because, similar to zeolite beta, one of the ordered intergrowths, with an alternating stacking of channels to the left then right gives a 12-ring spiral channel in the third direction. Consequently in order to synthesise this polymorph it will be necessary to control the surface nucleation, most probably via templating. The strategy to adopt will be to consider the growing surface topology then design a template that will both lock in to this surface then direct the next layer to grow to the left then right producing a spiral. A chiral molecule will be required but long spiral molecules are inappropriate as they will only ®ll the spiral channel and not lock into the growing surface. Second, to control the defect density. There are reasons why it is desirable to both increase and decrease the density of the open pore defects. Increasing the density will create a material with many ultra-large pores, decreasing the density may improve the stability of the material. The key is in increasing or decreasing the rate of nucleation at the surface relative to the propagation of a new layer across the surface. Increasing the nucleation rate will increase the density of defect sites and vice versa. Strategies to be adopted may include altering the temperature or supersaturation in the synthesis gel, adding surface binding agents to
8
M.W. Anderson et al. / Microporous and Mesoporous Materials 48 (2001) 1±9
Fig. 12. HREM of ETS-10 showing the layer growth which results in both the defects, which are the double pores, and the intergrowth structure seen as a random stacking of layers.
slow down surface nucleation, changing the source of nutrient etc. Clearly, there is still a long way to go to be able to control the individual growth processes within one crystal system, however, again knowledge of the mechanism of the growth processes allows the development of suitable synthesis strategies.
5. Growth units It is important to stress in this work that our microscopy studies do not reveal information about individual growth units. It will be necessary to perform associated spectroscopic studies to glean information in this area. However, our work does indicate that the growth process is not continuous, i.e. there are certain closed structures which are preferable and the growth is a stepwise mechanism. In this manner we are able to break down the process ®rst into layers, then sub-divide the layers into growth at edge sites and kink sites. The nature of the closed structures is unclear and theoretical studies such as those performed on calcite by Leeuw and Parker will be necessary [13].
However, we may speculate that the closed structures are probably complete cages such as sodalite cages or double six-rings etc. It should also be stressed that the terms edge and kink sites should be considered in a somewhat loose sense and not in the strict de®nition normally associated with crystal growth in dense phases. In a framework structure surface, edge and kink sites will be sites with increasing numbers of attachment points for the next closed structure.
6. Conclusions In conclusion we have shown that generally many framework structures grow via a layer growth process. Templating occurs by a surface decoration whereby the organic structure-directing agents locks into the growing face and keys the growth of the subsequent layer. Defects and intergrowths on framework structures are a consequence of the speci®c growth process and in principle these could be controlled by modifying the rates of individual growth processes. AFM as a
M.W. Anderson et al. / Microporous and Mesoporous Materials 48 (2001) 1±9
tool is able to reveal the relative rates of individual growth processes. Acknowledgements We would like to thank EPSRC (especially Advanced Fellowship AF/990985 to JRA), British Council, Daiwa Award and CREST (JST) for funding.
References [1] M.W. Anderson, J.R. Agger, J.T. Thornton, N. Forsyth, Angew. Chem. Int. Ed. Engl. 38 (1996) 1210. [2] J.R. Agger, N. Pervaiz, A.K. Cheetham, M.W. Anderson, J. Am. Chem. Soc. 120 (1998) 10754. [3] M.W. Anderson, J.R. Agger, N. Pervaiz, S.J. Weigel, A.K. Cheetham, in: Treacey, et al. (Eds.), Proc. 12th Int. Zeolite Conf., Baltimore, July 1998, MRS, 1487.
9
[4] S. Sugiyama, S. Yamamoto, O. Matsuoka, H. Nozoye, J. Yu, Z. Gaugshang, S. Qiu, O. Terasaki, Micorporous Mesoporous Mater. 1 (1999) 28. [5] T. Ohsuna, O. Terasaki, V. Alfredsson, J.-O. Bovin, D. Watanabe, S.W. Carr, M.W. Anderson, Proc. R. Soc. Lond. A 452 (1996) 715. [6] V. Alfredsson, T. Ohsuna, O. Terasaki, J.-O. Bovin, Angew. Chem. Int. Ed. Engl. 32 (1993) 1210. [7] T. Ohsuna, O. Terasaki, Y. Nakagawa, S.I. Zones, K. Hiraga, Phys. Chem. B 101 (1997) 9881. [8] O. Terasaki, T. Ohsuna, V. Alfredsson, J.-O. Bovin, D. Watanabe, S.W. Carr, M.W. Anderson, Chem. Mater. 5 (1993) 452. [9] S.M. Kuznicki, US Patent 4853202 (1989). [10] M.W. Anderson, O. Terasaki, T. Ohsuna, A. Philippou, S.P. MacKay, A. Ferreira, J. Rocha, S. Lidin, Nature 367 (1994) 347. [11] M.W. Anderson, O. Terasaki, T. Ohsuna, P.J. O'Malley, A. Philippou, S.P. MacKay, A. Ferreira, J. Rocha, S. Lidin, Philos. Mag. B 71 (1995) 813. [12] T. Ohsuna, O. Terasaki, D. Watanabe, M.W. Anderson, S. Lidin, Stud. Surf. Sci. Catal. 84 (1994) 413. [13] N.H. de Leeuw, S.C. Parker, J. Phys. Chem. B 102 (1998) 2914.