Growth Mechanisms in SAPO-34 Studied by White Light Interferometry

May 28, 2010 - Manchester M13 9LP, U.K., and §University College London, Department of Chemistry, Third ... natural chabazite (IZA structure code CHA...
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DOI: 10.1021/cg9016118

Growth Mechanisms in SAPO-34 Studied by White Light Interferometry and Atomic Force Microscopy

2010, Vol. 10 2824–2828

Børge Holme,*,† Pablo Cubillas,‡ Jasmina Hafizovic Cavka,† Ben Slater,§ Michael W. Anderson,‡ and Duncan Akporiaye† †

SINTEF Materials and Chemistry, P.O. Box 124, Blindern, NO-0314 Oslo, Norway, Centre for Nanoporous Materials, School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9LP, U.K., and §University College London, Department of Chemistry, Third Floor, Kathleen Lonsdale Building, Gower Street, London WC1E 6BT, U.K.

‡

Received December 22, 2009; Revised Manuscript Received May 10, 2010

ABSTRACT: The combined use of white light interferometry (WLI) and atomic force microscopy (AFM) revealed pentagonal growth spirals on the surface of SAPO-34 crystals. Detailed considerations of the crystal geometry and preferred step energies may explain the unusual shape of these growth spirals. Combining WLI and AFM is an efficient method for screening and detailed analysis of growth hillocks on crystals larger than 10 μm. The small pore silicoaluminophosphate molecular sieve SAPO-34 was first reported by Union Carbide in the 1980s.1,2 It is known to be one of the best catalysts in the conversion of methanol to olefins (MTO).3-6 SAPO-34 gives a very high selectivity (>80%) to light olefins (C2-C4), with almost 100% conversion of methanol. This is mainly attributed to its mild acidity and shape selectivity due to a small pore entrance. The main problem associated with SAPO-34 is the rapid deactivation due to coke formation during the MTO reaction.7,8 To prolong the lifetime of SAPO-34, it is necessary to develop catalysts which are more resistant to coke formation. A lot of research work has been done on this subject.9-11 Some of the parameters that influence the stability of this catalyst are the following: the amount and distribution of the silicon incorporated in the framework; the choice of structure directing agent (template) used during the synthesis; and the crystal size. Samples with relatively low silicon content show higher stability and increased acid strength of the catalytic sites. The amount and distribution of silicon depends on the choice of template.9,10,12 Regarding the effect of crystal size, it was found that crystals below 500 nm show the best catalytic performance.13 In order to prepare catalysts with optimal performance, a better understanding of the mechanism of crystal growth is needed. By revealing details of its nucleation and crystallization, it may be possible to synthesize materials with optimal pore architectures, functionality, morphology, and crystal size. The crystal structure of SAPO-34 is analogous to that of natural chabazite (IZA structure code CHA). Chabazite crystallizes in the trigonal crystal system with typically rhombohedralshaped crystals that are pseudocubic. The CHA framework is constructed from double 6-rings (D6R units), joined together by 4-rings enclosing ellipsoidal cavities. Each cavity is linked, in three nearly orthogonal directions, to neighboring cavities by six 8-ring windows. The cavities are accessible along the rhombohedral unit cell axes through the 8-ring windows (see Figure 2b and c).14,15 The rhombohedral unit cell dimensions for SAPO-34 are 0.94 nm.16,17 Since its invention,18 atomic force microscopy (AFM) has been a widely used technique in crystal growth studies. This is because AFM provides nanometer-scale resolution of surface features (down to 0.1 nm in the vertical scale) and it can measure in situ dissolution and growth processes. AFM studies in the field of nanoporous materials have gained momentum in the last several *E-mail: [email protected]. Telephone: þ47-98383946. Fax: þ4722067350. pubs.acs.org/crystal

Published on Web 05/28/2010

years due to the increasing need to understand the fundamental processes that control the growth of these materials. Originally, investigations focused on natural and synthetic zeolites,19-23 but recently a number of studies have been published on zeotypes24-27 and metal organic frameworks.28 White light interferometry (WLI) in the mode called vertical scanning interferometry (VSI) has become a valuable tool in the study of growth processes of crystals due to its high vertical resolution and ability to rapidly image much larger areas than can be done by AFM.29,30 WLI is particularly useful in combination with AFM when the complementary aspects of the two techniques can be utilized.31 So far, WLI has mostly been used to study the growth of geological samples. Few studies are available where WLI has been used for investigating nanoporous catalyst materials. As opposed to AFM, where an ultrasharp mechanical stylus scans the sample to obtain a topographic image of the surface, WLI is a noncontact technique based on interference of white light for measuring the surface topography.32 It takes typically 10-60 s to make one image, independent of the image width, which can vary between 50 μm and 5 mm, depending on the lens combination used. The vertical resolution, defined as the smallest step that can be measured, is on the order of 10 nm for the VSI mode. The lateral resolution in WLI is the same as for optical microscopes, i.e. 0.6 μm for the 50 lens used here.33 The accuracy (correctness) of a WLI instrument can be checked by measuring a calibration step and can be as good as 10 nm for a step height of 10 μm, or 0.1%, provided the standard has been independently measured with a similar accuracy. The precision (repeatability), defined as the standard deviation of repeated step height measurements on a glass standard of 10 μm height, is also about 10 nm with the instrument used here.34 Although WLI can be used to image almost any surfaces with a reflectivity above 2%, there is a limit to the tilt angle for specular surfaces. If too much of the light intensity is reflected away, and too little returns through the objective lens, there is not enough signal to determine the height at that point. Such regions with “bad pixels” will appear white in the images below. The background for this work is to obtain a fundamental understanding of the impact of synthesis parameters on crystal growth mechanisms for SAPO-34 using various “model” conditions. The aim of the current investigation was to study the surface topography and growth mechanisms of SAPO-34 crystals synthesized in the absence of HF. As an efficient way of achieving this goal, WLI was used as a screening tool for finding suitably oriented and scientifically interesting crystals for subsequent detailed analysis by AFM. r 2010 American Chemical Society

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Figure 1. SEM image of a random sample of grains from the studied catalyst powder. The rhombohedral grains often have small outgrowths of twinned crystals on the sharpest edges. Some crystals have a different morphology with hexagonal plates on the outer surfaces. This second phase made up less than 10% of the sample volume and was not further investigated in the present study.

Synthesis of SAPO-34: The SAPO-34 crystals were synthesized in the absence of HF according to the description in ref 35. The gel composition (mole ratio) used was 2.1 morpholine/0.3 SiO2/1.0 Al2O3/1.0 P2O5/60 H2O. The synthesis time and temperature were 48 h and 200 C (473 K), respectively. The solutions were prepared by first mixing 4.6 g of deionized water and 4.3 g of 85 wt % phosphoric acid (Merck) together with 2.6 g of the aluminum source (Catapal-B, Vista Chemicals). The solution was then stirred well before further addition of 4.6 g of deionized water. This mixture was labeled solution A. Solution B was prepared by mixing 1.1 g of Ludox LS-30 (Du Pont), 3.4 g of morpholine (Janssen Chimica), and 4.6 g of deionized water. This mixture was then slowly added to solution A during intense stirring, before 4.6 g of deionized water was added. The resulting gel was transferred to a Teflon-lined steel autoclave prior to heat treatment. Following the synthesis, the autoclave was cooled rapidly in cold water, and the resulting product was filtered and washed three times with deionized water. The powder obtained was then dried at 100 C overnight. The phase purity of each sample was checked by powder X-ray diffraction. Small amounts of a second phase were visible in the X-ray spectra. This phase was also seen in the SEM images, but the volume fraction was found to be below 10%. Samples were prepared for microscopy by partially embedding the crystals in crystalbond-509 thermoplastic (Electron Microscopy Sciences, USA), so the top surfaces remained exposed for AFM analysis. The WLI instrument used in this study was a WYKO NT-2000 (Veeco, USA) equipped with a motorized xy-stage for easy lateral positioning of the sample. All images were made using the vertical scanning interferometry mode. In order to improve the vertical resolution through reducing the noise level, ten images were made automatically in succession and the averaged image calculated. Two AFMs were used to analyze the crystals: (1) A Dimension 3100 with a NanoScope IIIa controller (Veeco, USA), operated in tapping mode using a μmash NSC15/AlBS cantilever with a resonance frequency of 288 kHz. The scan rate for large images 2 (20 μm) was 1 Hz, and 2 Hz was used for the smaller images. Default values were used for the other microscope parameters. (2) A Nanowizard II (JPK Instruments, Germany), operated in contact mode with SN type cantilevers (Veeco probes) with a nominal spring constant of 0.06 N/m. Scanning electron microscope (SEM) images were made using a JEOL JSM-5900LV operated at 30 kV with an 8 mm working distance and a chamber pressure of 13 Pa. The catalyst powder was sprinkled onto a glass plate which had been covered by

Figure 2. (a) SEM image of a SAPO-34 crystal with some smaller crystals and twins protruding from the surface. (b) Simplified drawing of 3 by 3 unit cells seen down the [001] direction and delimited by {100} faces. A unit containing 2 by 2 double 6-rings (D6Rs) is highlighted in red. The D6Rs are tilted in this projection, sloping up toward the lower left corner of the drawing. (c) A three-dimensional drawing of 2 by 2 by 2 D6Rs, showing the tilted orientation of the rings relative to the {100} faces.

carbon to achieve electrical conductivity. The catalyst crystals were not coated. Figure 1 shows a representative SEM image of the SAPO-34 sample studied. The grain size goes up to about 60 μm. Many of the grains have smaller twins along the edges. These give characteristic “ears” on the acute corners when the grain is aligned as in Figure 2a. The schematic drawing in Figure 2b shows the CHA framework type in an Æ001æ projection, referred to the rhombohedral unit cell. The similarity with the crystal morphology, shown in part a, indicates that the crystal surfaces are {100} planes. The 3D drawing in part c shows that the D6R building units are tilted relative to the {100} faces. This introduces a symmetry break in the (001) plane of the crystal where there is mirror symmetry along the short diagonal but not along the long diagonal (see Figure 5 for details). The sample prepared for WLI and AFM contained a total of 154 catalyst grains, as counted from an optical microscope image of the entire sample. Nineteen of these grains were found by WLI to have crystal facets with an approximately horizontal surface. Of these 19 SAPO-34 crystals with an orientation suitable for AFM, the grains with the most interesting topographic features based on the WLI investigation were selected for AFM analysis. Figure 3a shows the WLI topography image of such a grain, where several growth hillocks are clearly visible. The AFM images in Figures 3b and 3c correspond to the black and white squares in Figure 3a, respectively. The relative orientations of the images are also given by the crystallographic axes. On the lowmagnification WLI image, the growth hillocks look roughly triangular with the three pyramid edges pointing in the [100], [010], and [110] directions. However, the higher resolution images

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Figure 3. (a) WLI topography image of a catalyst grain with several growth hillocks visible on the surface. The black and white squares show the positions of the AFM images below, with the arrow pointing up. The AFM phase images in parts b and c show two growth hillocks with a pseudopentagonal shape. The corresponding crystallographic directions are given by the axes in all three images. They show that the terrace edges on the right-hand side of the hillocks lie along the [100] and [010] directions.

by AFM reveal that the growth spirals actually have a pentagonal shape. The steps delimiting the growing terraces are parallel to the [100], [010], [110], and [110] directions. The 94 corner formed by the [100] and [010] terraces generates the edge of the pyramid pointing in the [110] direction. Additionally, it can be seen in both AFM images that the growth hillocks are produced by composite spirals, with more than one dislocation present. Figure 4 shows the WLI and AFM investigation of a SAPO-34 crystal with only one dominating hillock on the (001) facet. In this grain the pentagonal shape of the hillock is more clearly visible. The axes in Figure 4a have been oriented to follow the convention from Figure 3 that the three sharpest edges of the hillock point along the [100], [010], and [110] directions. This becomes even more evident in the low-magnification AFM image (Figure 4b), where the pentagonal shape is outlined. The higher magnification AFM image in Figure 4c shows further details around the top of the hillock. Again, the terraces are parallel to the [100], [010], [110], and [110] directions. As seen also in Figure 3, the straightest terrace edges are the ones parallel to [100] and [010], whereas the other terraces tend to be a bit more rounded or rugged. Consequently, the sharpest (least rounded) corner is the one between the [100] and [010] terraces. The cross-sectional depth profile in Figure 4d shows steps in the surface along the dotted line in Figure 4c. The average terrace height is 0.9 ( 0.1 nm, which corresponds well to the repeat distance between layers of D6Rs, which is 0.94 nm (Figure 2). This indicates that the stable termination of the structure is the D6R, which parallels what has been observed on STA-7, whose structure is also made up of D6R in different orientations.27 The growth hillock is actually a composite spiral. The individual dislocations are visible as small dark dots in Figure 4c at the

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Figure 4. WLI and AFM analysis of a SAPO-34 grain with one dominant hillock on the surface. (a) WLI image of the complete crystal, where the hillock is clearly seen. It rises 130 nm up from the outer rim of the crystal surface. (b) AFM error image from the white frame in part a showing that the growth spiral of the hillock has a pentagonal shape, whose steps are parallel to the highlighted directions. (c) Higher magnification AFM height image from the center of part b where the peak of the hillock is shown in detail. The spiral is of the composite type, with multiple dislocations aligned. (d) Depth profile along the dotted blue line in part c showing steps with height between 0.9 and 1.0 nm.

positions where a new step appears. At least seven such dislocations can be found in the AFM image separated by distances ranging from 0.06 to 0.19 μm. Some of the dislocations have a Burger’s vector of 1.9 nm, hence producing two substeps. Another interesting feature of these complex spirals is that all have the same sign, so no closed loops developed—although this is probably coincidental. Since the dislocations are all operating as one complex source, the separation between them must be at least equal to 9.5rc, where rc is the radius of the critical 2D nucleus.36 Therefore, the critical radius is at least 20 nm. Although no values of the critical nucleus have been reported for zeolitic materials, the fact that complex spirals dominate the growth over the whole crystal (i.e., it dominates over possible single dislocations of one or additional steps) probably means that supersaturation conditions were close to equilibrium when the crystals were removed from solution.37 The origin of the composite spirals is not completely understood, but it may be due to the incorporation of small crystallites or other impurities, as has been shown in other systems.37,38 Our SEM analysis in fact shows crystals with small crystallites partially embedded on them (Figure 2a). Figure 5a shows a simplified SAPO-34 structure delimited by the {100} faces, with a hypothetical “island” of pentagonal shape highlighted in red. The figure also gives an indication why the terraces along [100] and [010] tend to be straighter and form the sharpest corner: Along these two directions, the neighboring D6Rs have the minimum distance, since these are “close packed” directions with regard to D6Rs. The terraces along Æ110æ type directions, on the other hand, are actually full of kinks on an atomic scale. It would be energetically more favorable for the system to simply extend a kink already present on a Æ110æ type terrace than to form a new kink along a straight Æ100æ type step. These growth spirals, with their apparently symmetry-breaking pentagonal shape, are quite unusual. One could, for instance, expect to see spirals bound by Æ100æ steps giving a rombohedral morphology, such as in calcite,39 which belongs to a similar space group. Nevertheless, this unusual shape may be explained by the

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Figure 5. (a) Drawing of the (001) crystal surface of the CHA framework type (pink) with a pentagonal island of D6Rs added (red). The pentagon follows low-index directions, as shown by the blue lines and vector indices. The inset is an AFM image from the same hillock as in Figure 4 and shows that the pentagonal shape of the hillock matches the low-index planes. Drawings b, c, and d are side views of the structure projected onto the vertical planes shown by the lines and corresponding letters in part a. The drawings show that the D6Rs form symmetrical steps along the [110] direction (b). Along the directions [010] (c) and [110] (d), the steps are asymmetric with acute and obtuse angles on opposite sides of the island.

tilting of the D6R units with respect to the crystal faces. This tilting can be observed more clearly on the three projected cross sections shown in Figure 5. Figure 5b shows how the steps parallel to the [110] direction are symmetrical on both sides. This is not the case for those parallel to the [010] and [110] directions, where one side is acute and the other obtuse. If, for example, the island was bounded by two [010] steps—one delimiting the left boundary and the other the right one—the step on the left would be acute and the one on the right obtuse (as can be seen in Figure 5c). So, even if they were parallel to the same direction, the steps would be structurally different. A similar situation is observed on calcite, which gives rise to anisotropic growth and dissolution.40 This alternation between acute and obtuse step geometry may be the reason why the terraces do not have a rhombohedral shape. Instead, they produce a spiral delimited by both Æ100æ and Æ110æ type steps. It may be that an obtuse [110] step is more stable than the obtuse Æ100æ steps; therefore, the terraces will be parallel to the former on one side of the spiral. On the other hand, the acute Æ100æ steps will be more stable, and therefore, the spiral will be delimited by the steps parallel to [100] and [010] instead of a single, acute [110] step. Since the absolute orientation of the crystal is not known, the tilt of the D6Rs could be reversed to the one described. In that case, the stable terminations will be the opposite. To gain further insight into the peculiar spiral geometry, one can consider the different step energies. For Æ100æ steps, two dangling bonds are exposed per D6R, whereas a Æ110æ step has four. This suggests that their stability will be very different. Atomistic simulations41 have shown that surface energy (and hence surface stability) is inversely correlated with the number of dangling bonds expressed on the surface, implying that Æ100æ steps are more stable than Æ110æ steps. We have carried out atomistic simulations using the siliceous CHA analogue of the SAPO-34 structure, which mimics the structural features of SAPO-34.27 Using a newly implemented version of the REaxFF42 force field and the GULP code,43 we found a formation energy of þ0.23 eV/A˚ for the regular Æ100æ step. The cost of making the jagged Æ110æ step is þ0.59 eV/A˚, more than twice that of the straight Æ100æ step. The step formation energies indicate that the jagged Æ110æ step edges are intrinsically unstable with respect to the straight Æ100æ step edges, and therefore, in the spirit of Gibbs theorem, one expects that straight Æ100æ step edges will be preferentially expressed on the crystal. This finding emphasizes the unexpected nature of the geometry around the growth spiral, which displays [110] and [110] step edges in addition to Æ100æ

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steps. However, the result is consistent with a hypothetical [110] step on the lower left side of the island in Figure 5a being replaced by [100] and [010] steps. This makes the step 1.4 times longer, but with a 53% reduction in the total step energy, based on the above values. The reason for the observed asymmetry is still not clear, but it may be that the template molecule (morpholine in this case) prefers to “sit” in a particular step geometry with the effect of stabilizing those steps. Preference of template molecules for particular step positions was demonstrated through molecular dynamics calculations on the STA-7 structure, where acute/obtuse steps are also formed due to the D6R tilt with respect to the surface.26 Thus, the peculiar pentagonal shape of growth spirals on {100} surfaces of SAPO-34 crystals is not yet fully understood. However, the interplay between energy minimization and growth kinetics results in relatively straight steps along Æ100æ directions and more curved steps approximately parallel to Æ110æ directions. Further work is needed to understand the spiral geometry in detail. The shallow hillocks formed by growth spirals are generally not visible by the optical microscopes associated with AFM instruments but can be detected by white light interferometry. Using WLI to select interesting grains for study by AFM, and using the WLI maps to locate the dislocation spirals, greatly reduced the time needed to make relevant AFM images for this study. WLI and AFM are complementary topographical imaging techniques that constitute a powerful combination for screening and detailed study of any grainy material with nearly flat surfaces. Due to the limited lateral resolution of WLI (0.6 μm), it is difficult to resolve features on crystals smaller than about 10 μm. Thus, the combined technique is most useful for crystals with facets much larger than 10 μm, where imaging the entire surface by AFM is time-consuming. Acknowledgment. The authors would like to thank the Research Council of Norway, SINTEF Materials and Chemistry, EPSRC, and ExxonMobil Research and Engineering for providing funding for this project.

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