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In-Situ Atomic Force Microscopy Study of the Dissolution of Nanoporous SAPO-34 and SAPO-18 Rachel L. Smith,† Jasmina H. Cavka,‡ Anna Lind,‡ Duncan Akporiaye,‡ Martin P. Attfield,† and Michael W. Anderson*,† †

Centre for Nanoporous Materials, School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, U.K. SINTEF Materials and Chemistry, P.O. Box 124, Blindern, 0314 Oslo, Norway

‡

S Supporting Information *

ABSTRACT: In-situ atomic force microscopy has been used to investigate the dissolution behavior of industrially relevant silicoaluminophosphate catalysts SAPO-34 and SAPO-18. Spiral growth is prevalent on these materials and it is common for the spirals to be composites of multiple dislocation sources. The spirals dissolve via classical step retreat and the structure dissolves in a two-step process via unstable intermediates. The data support the proposition that the terminating surface of SAPO-34 is composed of double 6-rings. SAPO-34 and SAPO18 both dissolve by removal of the same structural units with similar mechanisms.

1. INTRODUCTION Atomic force microscopy (AFM) has been at the forefront of crystal growth studies since its invention in 19861 and in 1996, AFM was used for the first time to investigate crystal growth in a zeolite.2 AFM has now been used to give insight into the crystal growth of a number of nanoporous materials including zeolites,3−7 zinc phosphates,8,9 and metal−organic frameworks10−12 (MOFs). Although much information can be obtained by studying the crystals postsynthesis, with appropriate quenching procedures, it is preferable to perform realtime growth studies to observe the crystal growth in-situ. This is relatively facile with MOFs and zinc phosphates due to the ability to grow the crystals from clear solutions at low temperatures (i.e., less than 70 °C).9,13 However, zeolites and silicoaluminophosphates (SAPOs) are extremely difficult to study in this manner owing to the opaque gels formed in the synthesis and the high temperatures required (typically greater than 150 °C). In-situ growth is only possible for zeolites that can grow from clear solutions and there is only one report as such.14 For gel systems an alternative approach is to work in undersaturated conditions to dissolve the crystals in-situ to examine this reverse process as a means of getting insight into the original growth process. In this study, we investigate the in-situ dissolution of screw dislocations in crystals of silicoaluminophosphates SAPO-34 and SAPO-18. Both materials are active catalysts for the methanol-to-olefins (MTO) process and there is extensive literature on their catalytic properties and performance.15−17 SAPO-34 is the industrial catalyst for the MTO process, which was recently commercialized in China.18 SAPO-34 has the CHA framework topology (Figure 1a) and SAPO-18 has the © XXXX American Chemical Society

Figure 1. (a) 3D representation of the CHA structure showing the D6Rs in gray and the CHA cages in orange. (b) 3D representation of the AEI structure showing the two orientations of the D6Rs in gray and white and the two orientations of the AEI cage in light and dark blue.

AEI framework topology (Figure 1b), as designated by the International Zeolite Association.19,20 Both structures are related by a common repeat layer of tilted double 6-rings (D6R, shown in gray in Figure 1), which are connected by fourmembered rings to build the framework. The stacking of the common repeat layer in CHA follows an AAA··· sequence (Figure 1a) whereas the stacking of the layers in AEI is in an ABAB··· sequence (Figure 1b). The common repeat layer has a layer height of 0.9 nm. The SAPO-18 unit cell contains two common repeat layers and thus has a 1.8 nm height. SAPO-34 is in the hexagonal crystal system and can be described by either a hexagonal or a rhombohedral unit cell. The rhombohedral unit cell is often a more appropriate description Received: October 5, 2015 Revised: November 8, 2015

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DOI: 10.1021/acs.jpcc.5b09710 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C based on the crystal habit.21 In the rhombohedral unit cell for SAPO-34 the cell faces are delimited by the tilted D6Rs with cell dimensions a = b = c = 9.459 Å and α = β = γ = 94.07°. AFM studies have shown that spiral growth from screw dislocations is very common in nanoporous materials and such growth has now been observed on zeolites,3 zinc phosphates,8,9 and MOFs.10−12 Silicoaluminophosphates have a high propensity for spiral growth and such dislocations were observed on SAPO-34,21 STA-7,22 and SAPO-18.23 Spiral growth is apparently facile in these materials even though there are necessarily large Burgers vectors associated with these structures (1−2 nm) with the potential for lattice strain at the screw core. Previously, ex-situ AFM studies were used to examine the growth mechanisms in these materials and to investigate the unusual shapes of the spirals. We recently showed that the spiral growth pattern on SAPO-18 is particularly complex.23 SAPO-18 exhibits platelet morphology with large (001) faces bound by {110} edges (Figure S1). The Burgers vector of the dislocation is 1.8 nm, i.e., one unit cell in the c-direction. The A and B layers in the SAPO-18 structure have different growth rates in each of the {110} directions due to the orientation of the large internal cages relative to the crystal facets. This results in interlaced steps along ⟨100⟩ toward the corners of the crystal face. The growth is also anisotropic in the ⟨100⟩ direction because the unit cell of SAPO-18 has a 95° tilt along the a-direction, producing an acute and an obtuse step edge. This in turn is reflected in slow growth along [1̅00] and the resulting extra step edge gives the spiral an overall triangular appearance.23 Spiral growth was also observed on SAPO-34.21 The crystals have rhombohedral morphology delimited by {100} faces (using the rhombohedral cell description). The spirals have an overall pentagonal shape where the steps of the growing terraces are parallel to the [100], [010], [110], and [11̅0] directions. Steps are consistent with the unit cell length 0.94 nm and the termination was postulated as the tilted D6Rs, as shown in Figure 1.21 Here, SAPO-34 and SAPO-18 are investigated using in-situ dissolution to further probe the growth mechanisms by determining intermediate step heights and, hence, metastable structures that are important during dissolution. SAPO-18 is prepared using TEAOH as the structure directing agent, and two SAPO-34 samples are investigated, both prepared with morpholine but with different gel ratios and silicon sources. We show that all materials dissolve via similar mechanisms and the terminating surface is the double 6-ring units.

Table 1. Gel Ratios of the SAPO-34 Preparations preparation

SiO2

Al2O3

P2O5

morpholine

H2O

A B

1.08 0.3

1.0 1.0

1.06 1.0

2.09 2.1

66 60

morpholine and deionized water. This solution was slowly added to the first solution with stirring. Finally, deionized water was added. Preparation A used fumed silica as the silica source with an aging time of 24 h at 38 °C followed by 24 h at 200 °C.24 Preparation B used Ludox LS-30 as the silica source with a synthesis time and temperature of 48 h and 200 °C.21 Materials are named SAPO-34-A and SAPO-34-B, respectively. AFM measurements were performed in contact mode on a JPK Nanowizard II Bio-AFM mounted on an inverted Axiovert 200 MAT optical microscope. A scan rate of 1−2 Hz was used for ex-situ scans and a scan rate of 4.5 Hz was used for in-situ scans. Silicon nitride tips (Bruker probes NP-10) with a nominal spring constant of 0.58 N m−1 were used. In-situ experiments were performed in a JPK BioCell. Crystals were scattered onto an epoxy resin (on a glass cover slide) and cured at 60 °C. The crystals were subsequently immersed and imaged in a pH 5 phosphoric acid solution. Images were analyzed using the JPK Data Processing software, and in most cases the lateral force images were used. A line-fitting tool was applied to the images and individual terraces were flattened using a plane fit for cross sectional analysis.

3. RESULTS 3.1. Dissolution of SAPO-18. The growth features on SAPO-18 are complex and were described in detail in a previous communication that concentrated on ex-situ AFM analysis.23 The crystals exhibit spiral growth, with interlaced terraces in the [100] direction due to different growth rates for each A and B layer in the [110] and [11̅0] directions (schematic shown in the Supporting Information, Figure S1). The growth rate in the [100] and [1̅00] directions is anisotropic, resulting in an additional step edge. This gives a triangular appearance to the spiral (also visible in Figure 2). Movie M1 in the Supporting Information shows the in-situ dissolution of a SAPO-18 crystal with a single dislocation. Figure 2 shows the lateral force AFM images of the initial period of dissolution from 6 to 28 min. The bright spot in the center is an etch pit near the core of the dislocation. Initially, very small etch pits form on the terraces (Figure 2a−c) before the remainder of the terraces dissolve rapidly (Figure 2d,e). During dissolution the terraces on lateral force images appear very bright when there is a change in friction or chemical composition at the surface. This phenomenon has been observed before in much of our in-situ work on the growth and dissolution of framework materials, most notably zeolite L6 and zinc phosphate SOD.9 It occurs when there are chemical changes occurring at the crystal surface that result either in a change in friction or as a result of energy dissipation that is imparted to the AFM tip. Whenever this large lateral deflection is seen, it is associated with very rapid crystal dissolution (or growth) which, in turn, is indicative that the surface is unstable. This effect is particularly prominent in Figure 2d. The exposed terraces on the spiral were completely dissolved within 12 min. The underlying structure is undisturbed and dissolves in the next sequence (Figure 2g−l). The step heights observed during dissolution are described in section 4.

2. EXPERIMENTAL SECTION SAPO-18 was prepared as described by Smith et al.23 The molar ratio of the gel was 0.01 SiO2:1 Al2O3:1 P2O5:1 TEAOH:35 H2O. A mixture of phosphoric acid (85%), tetraethylammonium hydroxide (35%), and deionized water was prepared. The mixture was heated to 30 °C and Ludox AS40 and Pural SB were added under continuous stirring before aging at 30 °C for 2 h with stirring. The gel was heated to 165 °C with a heating rate of 5 °C/h and maintained for 72 h with rotation. Two batches of SAPO-34 crystals were prepared with different gel ratios, using morpholine as the template (Table 1). The amounts used are given in Table S1 (Supporting Information). Deionized water, phosphoric acid (85%), and Catapal B were mixed before further addition of deionized water. In a separate container, the silica source was mixed with B


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The Journal of Physical Chemistry C

Figure 2. Lateral force AFM images of SAPO-18 dissolution at 2 min intervals from 6 to 28 min (a−l). The width of each image is 1.5 μm.

Figure 3. Lateral force AFM images of SAPO-18 dissolution at 2 min intervals from 138 to 160 min (a−l). The width of each image is 1.5 μm.

After 40 min the dissolution rate slowed (Movie M1). This phenomenon is normal when using a static fluid cell as the solution concentration local to the surface increases as the crystal dissolves. This has the effect of decreasing the undersaturation near the surface and consequently slowing the dissolution. Figure 3 shows the dissolution at a later stage from 138 to 160 min. Here, the interlaced steps and paired terraces, visible in Figure S2a (Supporting Information), separate into equidistant steps of height 0.9 ± 0.1 nm (Figure S2b), which corresponds to individual A and B layers. This is a result of (i) the anisotropy of growth in the [110] and [11̅0] directions is no longer expressed and (ii) the individual terrace edges communicating with each other owing to the rate of migration of species between step edges being fast relative to the rate of

terrace retreat. If the growth anisotropy is small, then this will tend to be accentuated under condition close to equilibrium (as in Figure S2a) and masked under conditions away from equilibrium such as in Figure 3. There is an etch pit visible at the center of the spiral, and this expands during dissolution to reveal a spiral dissolution pit.23 Dissolution inside the etch pit leads to an increase in size of the spiral dissolution pit over time and this process can be observed in Movie M1. 3.2. Dissolution of SAPO-34-A. SAPO-34 synthesized by Preparation A (XRD shown in Figure S3, Supporting Information) gives crystals that exhibit rhombohedral morphology (Figure S4). AFM reveals isotropic spirals on the surface (Figure 4a). The steps are slightly ragged, indicating that the C


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mechanism is unaltered for low contact forces but the rate is increased. 3.3. Dissolution of SAPO-34-B. SAPO-34-B crystals exhibit spiral growth in a pentagonal pattern, with a prevalence of multiple screw dislocations with high surface density.21 Figure 6a shows a crystal with a total of 15 dislocations

Figure 4. Screw dislocations on SAPO-34-A: (a) AFM height image of a spiral ex-situ (scale bar 0.5 μm); (b) lateral force AFM image of spiral dissolved in Movie M2 with the six spirals highlighted in different colors (scan size 2.5 μm).

system had not yet reached equilibrium when the experiment was terminated (close to equilibrium the balance between growth and dissolution has the effect to smooth edges as prominences are dissolved and pits grown out). The spiral shown in Movie M2 is a composite of six screw dislocations with same-handedness, Figure 4b. The terraces dissolve by step retreat and the spirals unwind (Movie M2). Figure 5 shows a dissolution sequence from 108 to 130 min. The smallest spirals completely unwind and the terraces pass over these dislocations as the steps retreat. The arrow on Figure 5i shows one such dislocation where the terraces are dissolving across the dislocation. Impurities and defects also affect the step retreat during dissolution. The feature in the lower-left of the scan area (indicated by an arrow on Figure 5a) is a hole that formed because of an impurity or defect in the lattice. The terraces dissolved around this point and the hole expanded during dissolution. The hole can be seen clearly on the AFM image in Figure S5. In all cases, the AFM tip promotes dissolution (apparent from Figure S6), however, as has been observed before, the

Figure 6. Screw dislocations on SAPO-34-B: (a) lateral force AFM image of the spiral dissolved in Movie M3 with the 7 spirals and 4 Frank−Read loops highlighted (scan size 3 μm); (b) AFM height image of the center of the spiral prior to dissolution (scale bar 0.5 μm).

highlighted in different colors: 7 spirals of one-handedness and 4 pairs of screw dislocations with opposite-handedness acting as Frank−Read sources (shown in red and dark green at the center and teal and magenta on the upper-left). This produces a composite pentagonal spiral of clockwise handedness. There were many dark spots on the surface of the crystal when imaged ex-situ (Figure 6b), which are small etch pits possibly from partial dissolution at the end of the synthesis or during the washing step. The dissolution is shown in Movie M3. At the beginning of the experiment the terraces peel away very quickly similar to SAPO-18, but more rapidly in this case (6 min for complete terrace dissolution, Figure S7). After five to six layers the dissolution slowed into step retreat. The initial rapid dissolution

Figure 5. Lateral force AFM images of SAPO-34-A dissolution at 2 min intervals from 108 to 130 min (a−l). The width of each image is 2.5 μm. The arrow on (a) shows a surface impurity that disrupts the step retreat, and the arrow on (i) shows the steps retreating over a dislocation center. D


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Figure 7. Lateral force AFM images of SAPO-34-B dissolution at 10 min intervals from 56 to 126 min (a−l) showing the dissolution under the impurity (black spot). The width of each image is 3 μm.

Figure 8. Dissolution of SAPO-34-A. (a), (d), (g), and (j) show the lateral force AFM images. (b), (e), (h), and (k) show the cross section analysis of the lines drawn on the AFM images. (c), (f), (i), and (l) show a schematic of the respective heights during dissolution assuming a D6R termination. Lines on the cross section appear sloped, but this is an artifact of the low gains used during scanning.

4. DISCUSSION

was not observed in SAPO-34-A and may be promoted in SAPO-34-B by the etch pits on the surface (Figure 6). Impurities disrupt the terrace retreat in SAPO-34-B. The cluster of impurities on the right-hand side of the scan region (appears black in Figure 7a) dissolved to reveal small terraces underneath that had not dissolved at the same rate as the rest of the spiral (Figure 7b−f). This can be seen clearly in Movie M3. These small terraces dissolved faster than the rest of the spiral, but the step retreat at this point was disturbed for an extended period of time as visible by the elongated corner of the terrace in Figure 7h. The spiral eventually recovered and returned to the original shape.

It is clear from these in-situ AFM images that the dissolution occurs in at least two distinct stages that can be differentiated in the lateral force images by the presence or absence of large lateral deflection resulting in a bright area on the AFM image. The sequence of images in Figure 8 (SAPO-34-A) helps to interrogate these individual stages in the dissolution through the development of an isthmus that makes height analysis more facile. The region in question can be seen near the top of the dissolution image sequence shown in Figure 8. The crystals first dissolve from a terrace height corresponding to one unit cell of 0.94 nm (±0.1 nm, Figure 8a−c) to 0.75 nm (Figure 8d,e). This corresponds to removal of the D6Rs leaving the CHA cage intact (Figure 8f). During this stage there is no extra lateral E


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The Journal of Physical Chemistry C force observed as the dissolution step maintains closed cages. In the next step the terrace becomes very bright with a height of 0.38 nm (Figure 8g,h). This corresponds to breakdown of the internal CHA cage to the top of either the eight-membered ring (5.1 nm) or four-membered ring (3.1 nm, Figure 8i). The height of the 4MR is closest in height to the observed step height and is within the 0.1 nm error in the measurement. Both terminations contain unstable Q2 sites that results both in the large lateral force and in the ensuing very rapid dissolution owing to the unstable nature of the open cages. The final step shows terrace with height of 0.19 nm (Figure 8i−l). The same dissolution heights were measured for terraces on SAPO-34-B (not shown). The closed-cage termination on SAPO-34 was previously postulated as the D6Rs,21 and this termination is shown in the projection in Figure 8c. The experimental values corroborate this terminating structure. The CHA cages are also a possible closed-cage termination and both possibilities are shown in Figure 9. If the CHA cage were the terminating surface, the

Figure 10. (a) Enlarged image of SAPO-34-A showing the color difference across the terraces (bright at position 1 and dark at position 2). (b) Cross section across the line in (a) and the vertical dotted lines correspond to the triangles on (a). The difference across the terrace is 0.38 nm.

conclude what combination of supersaturation or silicon source is responsible. Despite these differences, both samples dissolve via the same mechanism with the same subunits. The stages of dissolution in SAPO-18 can be interrogated most easily with reference to the image shown in Figure 12. The unusual spiral dissolution mechanism in SAPO-18, characterized by a spiral pit close to the screw center, results in the isolation of individual growth terraces. A schematic of this process is shown in Figure 11. The terraces recede toward

Figure 9. Possible terminations on the (001) face of SAPO-34 shown (a) with and (b) without the D6Rs present. (a) The D6R termination gives two Q3 sites per unit area. (b) The CHA cage termination gives four Q3 sites per unit area.

possible step heights during dissolution would be 0.93, 0.67, and 0.48 or 0.17 nm. These values are within the 0.1 nm error of the AFM instrument. However, a good indication of the surface stability is the number of Q3 sites on the surface, where a lower Q3 count per unit area suggests a more stable surface.25 Figure 9a shows the termination of D6Rs, which has two Q3 sites per repeat unit. The CHA termination (Figure 9b) has four Q3 sites in the same area. This strongly suggests that the surface termination is the double 6-rings. The same conclusions can be drawn by analysis of the AFM image in Figure 10 recorded before the isthmus is formed. In this image, the retreating terraces show a banding between regions that exhibit large lateral force (bright, region 1 in Figure 10a) and low lateral force (dark, region 2 in Figure 10a). The terraces are receding in the direction of the white arrows, and therefore, the bright regions follow the receding terrace. The step height between region 1 and 2 is 0.38 (±0.1 nm) corresponding to an unstable, intermediate part of the structurethe 4MR or 8MR. Consequently, the rate of dissolution of the receding terrace is fast enough to expose areas that are not fully dissolved down to the next stable, closed-cage structure. SAPO-34-A and SAPO-34-B were prepared using different levels of silicon and different silicon sources. The terrace morphology was altered between these two samples (isotropic versus pentagonal spirals, respectively), and it is not possible to

Figure 11. (a), (b) Lateral force AFM images after 34 and 36 min, respectively. (c), (d) Schematic of the terrace patterns at the center of the spiral in (a) and (b), respectively, showing how the isolated terraces form during dissolution.

the center while simultaneously the spiral dissolution pit is dissolving outward from the core. As the step to the right of the spiral dissolution pit recedes (shown in green, Figure 11c), it connects with the outer step of the dissolution pit and the receding step splits into two. In this process, one side is reconnected to the spiral dissolution pit and the other forms an island of single-layer height (0.9 nm, half the SAPO-18 unit F


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18). Spiral growth is shown to be an important mechanism for growth in both systems. Closed-cage structures are shown to be much more stable than open-cage structures and the stable surface of both materials is composed of D6R units. However, because of the relative rates of dissolution of these two structure types it is possible to observe the intermediate, unstable, open-cage structures using AFM techniques and, in particular, lateral force microscopy. The dissolution sequence for both SAPO-34 and SAPO-18 for a single layer is identical; however, because the layer sequencing is different between SAPO-34 and SAPO-18, the nature of the screw dislocations is substantially different.

cell). The heights on these isolated terraces can then be easily probed. The step height was measured across the isolated terraces during the dissolution. Figure 12a,b shows magnified images of

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b09710. Amounts used in the SAPO-34 syntheses, XRD patterns, SEM images, AFM images, AFM step heights (PDF) In-situ dissolution of a SAPO-18 crystal with a single dislocation (MOV) Spiral on SAPO-34-A dissolved (MOV) Dissolution of SAPO-34-B (MOV)

Figure 12. (a) SAPO-18 terrace during the first step of dissolution. (b) Bright SAPO-18 terrace during the second step of dissolution. (c) Cross section across the line in (a), 0.7 nm. (d) Cross section across the line in (b), 0.4 nm. (e) Schematic of the SAPO-18 structure showing the possible heights assuming D6R termination.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

the center of the spirals in Figure 3e,f. The terrace first dissolves from the full 0.9 nm height to 0.7 nm (±0.1 nm, Figure 12c), corresponding to removal of the top of the double 6-rings (Figure 12e). The terrace then becomes very bright (large lateral force) with a height of 0.4 nm (±0.1 nm, Figure 12d), corresponding to one of the two possible unstable open-cage units. The process is identical to that observed for the SAPO-34 structure and reflects their structural similarities. The SAPO-18 dissolution was rapid at the beginning of the experiment and then slowed as the local concentration of the solution changed. The step heights recorded while the terraces were rapidly receding are also consistent with the breakdown of the structure described above (Figure S8). When the entire terrace starts to recede, as seen in Figure 2d, the step heights are 0.7 nm (Figure S8c). Then as only small, round terraces remain they appear bright with height 0.4 nm (Figure S8d). This rapid dissolution appears to be similar to the mechanism of dissolution on zeolite A.26 In the case of zeolite A, the D4Rs dissolved independently before concerted dissolution of the sodalite cages. One might expect that SAPO-18 may dissolve in a similar manner by dissolution of the D6Rs followed by removal of the complete AEI cages. However, in SAPO-18 the terraces are simultaneously receding while also dissolving down, thus intermediate heights are observed (ca. 0.4 nm). It has been shown that the closed cages are the rate-determining units in crystal growth of zeolites.25 If the relative rates of dissolution by terrace retreat and dissolution of the unstable open-cage structures is in the right proportion (as is the case here), then it is possible to observe the open-cage intermediate structures by AFM.

ACKNOWLEDGMENTS The authors thank the EPSRC and the Research Council of Norway for funding. REFERENCES

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5. CONCLUSIONS In conclusion we have shown the first in-situ AFM study on the dissolution of silicoaluminophosphates (SAPO-34 and SAPOG


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