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Nanoporous Intergrowths: how crystal growth dictates phase composition and hierarchical structure in the CHA/AEI system Rachel L. Smith, Wojciech A. Slawinski, Anna Lind, David S. Wragg, Jasmina H. Cavka, Bjørnar Arstad, Helmer Fjellvåg, Martin P. Attfield, Duncan Akporiaye, and Michael W. Anderson Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm504284x • Publication Date (Web): 14 May 2015 Downloaded from http://pubs.acs.org on May 23, 2015

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Chemistry of Materials

Nanoporous Intergrowths: how crystal growth dictates phase composition and hierarchical structure in the CHA/AEI system Rachel L. Smith,† Wojciech A. Sławiński,‡,1 Anna Lind,§ David S. Wragg,‡ Jasmina H. Cavka,§ Bjørnar Arstad,§ Helmer Fjellvåg,‡ Martin P. Attfield,† Duncan Akporiaye,§ and Michael W. Anderson*,†

† Centre for Nanoporous Materials, School of Chemistry, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK ‡ Centre for Materials Science and Nanotechnology, Department of Chemistry, University of Oslo, PO Box 1126, 0315 Oslo, Norway § SINTEF Materials and Chemistry, PO Box 124, Blindern, 0314 Oslo, Norway 1

On leave from Institute of Experimental Physics, University of Warsaw, Pasteura 5, 02-093 Warsaw, Poland

ABSTRACT Some of the most important nanoporous materials that are used for industrial applications are formed as intergrowths between structurally related phases. Further, the specific properties and functions are often strongly related to the nature of these intergrowths. By their nature such structures are notoriously difficult to characterize in detail and thereby formulate a structure/property relationship. We approach the problem of the industrially relevant CHA/AEI intergrowth system by not only getting insight into the structure of the materials but also the crystal-growth mechanism and show that the former is crucially dependent upon the latter. Through a detailed x-ray diffraction analysis with optimization of the CHA/AEI layer stacking sequence, it is shown that up to three distinct components are present. These consist of the two end member structures intimately co-crystallizing with an intergrowth structure. The intergrowth composition is further corroborated by NMR and unit cell measurements. The mechanism by which these complex intergrowth structures form is revealed by Atomic Force Microscopy that shows there are at least two competing mechanisms of growth at the surface; layer-by-layer and spiral. This has profound consequences on the resulting intergrowth materials, as intergrowth formation is not permitted in spiral growth. The competition from the lower energy spiral growth at screw dislocations does not allow intergrowth formation and consequently results in blocks of pure-phase AEI or CHA. Owing to this competitive growth nature, the different possibilities furnish the material with its higher-level hierarchical structure. longer lifetime, than SAPO-34.4,5 CHA/AEI intergrowths are

INTRODUCTION

reported in the patent literature as desirable catalysts for the Silicoaluminophosphate SAPO-34 is a widely used 1

MTO reaction and can be more stable to deactivation than

catalyst in the Methanol-to-Olefins (MTO) process. SAPO-

pure-phase SAPO-34.6-8 However, the structure of these

34 gives a high selectivity to light olefins (C2-C3) and high

intergrowths are difficult to characterize and are poorly

conversion rates due to the small pore aperture (3.8 Å) and

understood.

appropriate high acid site density.

2

One of the main

challenges with the SAPO-34 catalyst is rapid deactivation due to coke formation.3 The related material SAPO-18 is a potential MTO catalyst and typically has a lower acid site density and lower acid strength, hence a lower activity but

1 ACS Paragon Plus Environment


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complementary set of tools with the aim to elucidate the structure of these intergrowths in order to understand their improved catalytic properties. X-ray diffraction (XRD) has been used to determine the phase composition of the materials and with sophisticated Figure 1. Framework structure of a) SAPO-34, projection along

modeling methods, it was possible to quantify the level of

[110] and b) SAPO-18, projection along [100] showing the

intergrowth precisely. Solid-state nuclear magnetic resonance

orientation of the tilted D6Rs and the difference in layer

(NMR) spectroscopy revealed the local environment of

stacking and internal cage type.

different sites in the CHA and AEI structure. Peak fitting has enabled quantification of the level of intergrowth and we have

SAPO-34 has the same framework type, CHA, as the zeolite chabazite.9 The framework can be considered as layers of tilted double 6-rings (D6Rs), all with the same orientation, in an AAAA… stacking pattern (Figure 1a, grey). Here, these layers will be referred to as the common repeat layer. The stacking of these D6Rs gives rise to the large,

found good correlation between XRD and NMR data. Scanning electron microscopy (SEM) was used to determine the morphology of the materials and atomic force microscopy (AFM) allowed direct observation of the surface topography of the intergrowths. AFM is essential to understanding the growth of these CHA/AEI intergrowth structures. In the past, these techniques have been used in

internal CHA cage (Figure 1a, orange). The related material SAPO-18, framework type AEI, is built of the same common repeat layers but stacked in an ABAB… pattern, leading to an alternation in the orientation of the D6Rs in the [001] direction (Figure 1b, white and grey shows the alternating orientation of the D6Rs).10 This gives a different internal cage type, the AEI cage (Figure 1b, blue). The AEI cages also alternate in direction every half-unit cell due to the differences in the D6R stacking. Since the (001) surface of SAPO-18 is isostructural with SAPO-34, it is possible to form intergrowths of the two phases in the [001]AEI direction. However, these CHA/AEI intergrowths are not as well understood as the SAPO-34 and SAPO-18 end-members. It is important to understand the formation of these intergrowths in order to understand their improved catalytic activity for the

isolation on various zeolitic intergrowth materials. XRD is frequently used in conjunction with another technique but a combination of XRD, NMR and AFM is less common. XRD is a powerful technique for characterization of intergrowth materials because it usually gives the first indication that the material is not a pure phase. Intergrowth diffraction patterns are distinct from the combined patterns of each end-member and it is possible to accurately model these differences. Simulated diffraction patterns have been calculated using the DIFFaX11 and DIFFaX+12 programs for a number of years and these have been critical for investigation of a number of intergrowth systems.13-17 Very recently, we reported a model for characterizing CHA/AEI intergrowths using the DISCUS package.18 DISCUS allows multi-phase refinement and it is possible to accurately calculate the level of intergrowth in

MTO process. Here we present a carefully designed set of

materials with more than one crystalline phase. Previous studies on zeolite intergrowths typically

intergrowth samples where control of the silicon content in the materials allowed concomitant control of the ratio of CHA and

AEI

character

in

the

intergrowth.

With

this

comprehensive set of materials we have then employed a

used

XRD

combined

with

high-resolution

electron

microscopy (HRTEM and/or HRSEM) in order to investigate the stacking faults in crystals.16,17,19 HRTEM provides extremely detailed images of the layer stacking in nanoporous


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materials, but with no indication towards the mechanism of

Synthesis

of

materials.

The

materials

were

the formation of the faults and no surface detail. AFM is a

synthesized according to the synthesis route presented by

surface technique that allows analysis of the surface

Mertens et al.8 The changes in phase composition were

topography and the terrace step heights down to 0.1 nm

induced by varying the silicon content. The molar ratio of the

vertical resolution.20 The terrace patterns and step heights can

final gel was x SiO2:1 Al2O3:1 P2O5:1 TEAOH:35 H2O,

be directly related back to the framework structure to give

where x was varied between 0-0.3. A mixture of phosphoric

information about the growth mechanism. AFM has been

acid,

used to look at intergrowth materials in the past, but in those

(TEAOH, 35%, Sigma Aldrich) and deionized water was

cases only limited conclusions could be drawn due to the

prepared. The mixture was heated to 30°C and Ludox-AS-40

difference in morphology, or the similarity in surface

(40% SiO2, DuPont) and Pural SB (76% Al2O3, SASOL)

structure, of the end-members.

21,22

(85%,

Merck),

tetraethyl

ammonium

hydroxide

However, if the end-

were added under continuous stirring before aging at 30°C for

members in a series have distinct surface features or step

2 h under stirring. The aged gel was transferred to Teflon-

heights it is feasible to investigate the intergrowths by AFM.

lined autoclaves and heated to 165 °C with a heating rate of 5

Former NMR studies on SAPO-34 have typically

°C/h, and maintained for 72 h with rotation. The reaction was

focused on the silicon distribution in the framework, since

quenched in water and the product was separated by

23-26

NMR

centrifugation. The product was washed well with deionized

characterization of intergrowth materials is less common.

water and dried at 95 °C overnight. The final amount of

However, 29Si and 13C MAS-NMR have been used to study

silicon in the materials was verified by Energy Dispersive

FAU/EMT intergrowths, where deconvolution of the peaks

Spectroscopy (EDS). Materials were named Sx where x is the

27,28

percentage silicon content by weight, ranging from 0.0 to

If there are sufficient differences in the spectra of the two

7.0%. The list of the Si/Al ratios in the materials is given as

end-members in a series, then there is potential for

Table S1 in the Supporting Information.

this is key to understanding the catalytic activity.

allowed estimation of the relative amounts of each phase.

estimation of phase composition within intergrowths by peak fitting and deconvolution.

Characterization.

Synchrotron

radiation

(SR)

powder diffraction patterns were collected at the European

In this work, we report a combination of diffraction,

Synchrotron Radiation Facility (Grenoble, France). The

spectroscopy and microscopy to probe not only the short- and

Swiss-Norwegian beamline (SNBL) was used in high-

long-range order of these materials, but also the crystal-

resolution mode. Samples were calcined prior to analysis at

growth mechanism. This in turn has allowed us to understand

600 °C for 15 hours. All samples were measured at 250 °C in

how the complex intergrowths are formed and consequently

a 0.5-0.7 mm borosilicate glass open capillary to keep them

what types of structures are possible at the different length

dehydrated. The wavelength used was λ = 0.504 Å, though

scales. In addition, we show the importance of considering

for easier comparison with other studies the diffraction

crystal growth to understand the structure of these important

patterns shown in Figure 2 are plotted using the wavelength

industrially relevant materials.

from CuKα radiation. The scattering vector, Q, range measured was 0.4 < Q < 3.8 Å-1 but only a limited Q range was used in the refinement. SR powder diffraction data were analyzed using the computer program DISCUS.29,30 This

EXPERIMENTAL SECTION

program allows calculation of powder patterns for crystal structures built up of stacking faults. All the refinements were



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performed using the evolutionary algorithm implemented in

power decoupled MAS-NMR spectra were recorded with a

Diffev.31 Depending on the sample, a one, two or three phase

spin rate of 5000 Hz and a recycle delay of 10 s.

model was used. Each pattern was calculated as an average of

SEM was performed on an FEI Quanta 200 ESEM in

several individual patterns. This is due to the fact that each

high vacuum mode. Samples were dusted onto carbon tape

iteration of stacking-fault generation gives a different

and gold-coated prior to analysis. Images were collected in

stacking sequence and calculated powder pattern. Further

secondary electron mode with a working distance of 10 mm.

experimental and refinement details are described in Sławiński et al.18

AFM was performed in contact mode in air on a JPK Nanowizard II Bio-AFM mounted on an inverted Axiovert

The fraction of the CHA component in each material

200 MAT optical microscope. Silicon nitride tips (Bruker

was calculated from the XRD data and these calculations

probes NP-10, spring constant 0.58 Nm-1) were used with a

(below) relate to the discussion for Figure 11a. The fraction

scan rate of 1-2 Hz. Images were analyzed using the JPK

of CHA layers in the overall number of layers is given as n34.

Data Processing software. A line-fitting tool was applied to

For growth faults, n34 is equivalent to the Stacking Fault

the images and individual terraces were flattened using a

Rate (see Diffraction section for details). For displacement

plane fit for cross sectional analysis. Graphics of the cage structures were produced by in-

faults, n34 is calculated from the following equations;

house software using Mathematica 9.0.1. n34 = 1 – 2 * SFR * (1-SFR)

if 0 ≤ SFR ≤ 0.5

n34 = 2 * SFR * (1-SFR) if 0.5 ≤ SFR ≤ 1

RESULTS

The total number of layers is calculated using the following

A series of CHA/AEI intergrowth materials were synthesized

equation, where phase is either AEI, CHA or intergrowth and

by varying the silicon ratio in the synthesis gel. Typically,

fraction is the amount of each phase (given in Table 1);

low levels of silicon result in SAPO-18 and increased levels of silicon prompt SAPO-34 formation.15 A sequential increase

TOTAL=n34(phase1)*fraction1+n34(phase2)*fraction2+n34(

in silicon content from 0.0% to 7.0% silicon was used to

phase3)*fraction3

synthesize a series of materials with a gradual change from AEI to CHA character (final phase composition described

Solid-state MAS-NMR was performed on a Bruker Avance 400 MHz spectrometer using a 4 mm probe.

27

Al

spectra were recorded with a spin rate of 12000 Hz and a 31

recycle delay of 5 s. P spectra were recorded with a spin rate of 14000 Hz and recycle delay of 5 s.

29

below). The heating rate during synthesis was carefully controlled in order to prevent formation of SAPO-5 (AFI) as an impurity phase. Diffraction. The phase composition of the materials

Si spectra were

was determined by X-ray powder diffraction. Diffraction

recorded with a 60 s recycle delay in order to observe any

patterns of the calcined materials are shown in Figure 2. The

silicon islands. Spectra were recorded at a spin rate of 5000

pattern of the material with no silicon, S0.0, is consistent with

Hz using a 45° pulse tip angle. A 29Si NMR experiment using

that of the pure AlPO-18 structure. The diffraction patterns

a recycle delay of 60 minutes was tested to determine whether

changed over the range S0.5 to S1.0 as the level of the pure

60 s was sufficient and there were no differences between the

phase of the AEI structure decreased and the amount of

spectra collected with different recycle delay times. 13C high-

CHA/AEI intergrowth and CHA phase increased. For S1.7 and above, the intergrowth component was the primary phase,


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with SAPO-34 as a secondary phase. These changes were

from a change in stacking in the ABAB… sequence (Figure

evidenced by specific line shapes and/or broadening on a

3). Both types of faults give different characteristic diffraction

number of peaks and diffuse scattering between Bragg

patterns depending on the level of intergrowth.18 The level of

reflections.

intergrowth is quantified by the Stacking Fault Rate (SFR), where 0 describes a pure CHA phase and 1 describes a pure AEI phase. A SFR of 0.5 describes a completely random stacking of A and B layers.

Figure 2. Synchrotron X-ray diffraction patterns of materials S0.0-S7.0 as a function of silicon content.

The precise amounts of each phase in the diffraction patterns were calculated using the DISCUS program as described in Sławiński et al.18 The model allows for accurate

Figure 3. Crystal structure of SAPO-34 (left) with displacement

calculation of the level of stacking faults in the materials and

type stacking fault caused by insertion of a faulted layer (B

also the relative amounts of each phase. The intergrowths are

instead of A). AlPO-18 crystal structure (right) with growth type

described by the stacking faults between CHA (AAAA… or BBBB… stacking) and AEI (ABAB… stacking). The

stacking fault. CHA and AEI cages are shown in orange and blue, respectively.

diffraction patterns can be modelled according to two types of stacking faults: displacement and growth depending on the

Table 1. Relative fractions of phases in materials S0.0-S7.0.

silicon content. Displacement faults arise from a change in stacking in the AAAA… sequence and growth faults arise

Material


AEI

Intergrowth

CHA

(%)

(%)

(%)


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S0.0

100

-

-

Figure 4 shows the dimensions and volume of a

S0.5

84.1

15.9

-

layered unit cell. The basic unit cell expands when the silicon

S0.7

35.4

31.4

33.2

level increases, which is likely to be due to the replacement of

S1.0

13.1

43.9

43.0

phosphorus with the larger silicon atoms.

S1.7

-

64.0

36.0

S3.6

-

65.8

34.2

S4.8

-

63.0

37.0

S7.0

-

75.7

24.3

There is a common description of displacement and growth stacking fault types for both SAPO-34 and SAPO-18 based structures (Figure 3). For both intergrowth types, a stacking fault occurs when one layer of the ‘wrong’ type is inserted in the crystal, which then continues to grow in the same way as before (see the common repeat layer indicated by dashed lines on Figure 3). Table 1 shows the calculated amounts of AEI, CHA/AEI intergrowth and CHA phases in each sample, based

Figure 4. Layered unit cell values for materials S0.0 – S7.0. The

on the SFR. Here, a SFR of 0.98 gives AEI, SFR of 0.05

error bars are based on a statistical distribution of trial member

gives CHA and the intergrowth phase is given from a SFR of

group as described in Sławiński et al.18 and are strongly

0.27-0.5. The absence of silicon in the preparation affords

dependent on the refinement procedure.

pure AlPO-18. At low levels of silicon (S0.5 – S1.0), the amount of the intergrowth phase rapidly increases and the

NMR Spectroscopy. Solid-state

27

Al,

29

Si and

31

P

amount of the AEI phase decreases. Low levels of SAPO-34

NMR were performed to gain an insight into the structure and

are present in all materials except S0.0 and S0.5. Above S1.7 all

chemical composition of the intergrowths. Solid-state

the materials have a similar phase composition showing

spectra were obtained to see if it is possible to directly

mostly highly intergrown phase (above 60%) alongside pure-

observe the template interactions in the different framework

phase SAPO-34.

types.

A single AEI phase model was used for S0.0 material.

The

27

13

C

Al MAS-NMR spectra of the as-synthesized

A model consisting two phases was required for the majority

materials (Figure 5a) shows signals in the region of 30-50

of the samples and for S0.7 and S1.0 a three-phase model was

ppm, assigned to 4-coordinate Al, and in the region centered

necessary to model all features of the observed diffraction

at 9-11 ppm, assigned to 5-coordinate Al. The 5-coordinate

pattern, with the fitting matching extremely well with the

species arise from framework interactions with water or

observed diffraction patterns. Results from the detailed fits

template molecules. Two peaks were observed in the 4-

for the samples are given in the Supporting Information as

coordinate region in S0.0 and one in the 5-coordinate region,

Figure S1. Full details of the modeling for materials S0.0 and

corresponding to the three distinct Al crystallographic sites in

S4.8 were described previously.

18

AlPO-18.32 The sharp resonance at 46.0 ppm and the broader,


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neighboring peak at 40.3 ppm switch in intensity from S0.5 to S0.7. From S1.0 upwards, the resonance at 46.0 ppm reduced to a shoulder and the spectra began to resemble a typical SAPO-

a)

b)

c)

d)

34 pattern with a single resonance in the 4-coordinate region.33 29

Si MAS-NMR was performed on all as-synthesized

materials, with the exception of S0.0 due to the absence of silicon in AlPO-18. The typical Si(4Al) peak centered at -91.6 ppm is visible in all materials (Figure 5b). The absolute intensity of the Si(4Al) resonance increased as the silicon loading was increased. The presence of silicon islands, i.e. Si(4Si), is characterized by an additional peak at -110 ppm.24 Low levels of silicon islands, less than 10%, were observed in the two materials with the highest level of silicon: S4.8 and S7.0. S7.0 also has evidence of weak Si(2Al) and Si(1Al) resonancesError! Bookmark not defined. at -99.8 and -105.1 ppm. The 31P NMR spectra of the as-synthesized materials are given in Figure 5c. Three resonances were observed in S0.0 (AlPO-18) at -12.1, -29.0 and -30.0 ppm in a 1:1:1 ratio corresponding

to

the

three

distinct

phosphorus

crystallographic sites.32 The resonance at -28 ppm did not change in intensity as the amount of silicon was increased. The resonance at -12.1 ppm reduced to a minor peak and the resonance at -30.0 ppm reduced to a shoulder in S0.7 before

Figure 5. a) 27Al; b) 29Si; c) 31P and d) 13C MAS-NMR spectra of

disappearing under the larger, neighboring peak. There are

the as-synthesized materials.

additional, weak resonances surrounding the -12 ppm peak and these are attributed to extra-framework interactions. Pure-

13

C MAS-NMR was used to investigate the template

phase as-synthesized SAPO-34 is reported to show one

interactions in the as-synthesized materials. The structure-

resonance at -29 ppm.26,33 In AlPO-18, the three sites can be

directing agent (SDA) used in these materials was

distinguished but in SAPO-18 the two resonances at -30 ppm

tetraethylammonium hydroxide (TEAOH), which has two

overlap, giving two peaks in a 2:1 ratio (one at -29 ppm and

carbon environments at 51.3 and 6.4 ppm.34 Figure 5d shows

one at -12 ppm).

26

The resonance at -12 ppm, which

the

13

C MAS-NMR spectra of all as-synthesized materials

corresponds to the AEI structure, is clearly still present in

with each of the two resonances magnified. All spectra

Figure 5c even at the highest level of silicon (S7.0). However,

showed only two carbon environments from the TEAOH in

this is not a simple mixed phase sample and there is no AEI

the pores. However, there is a shift in peak position from 6.9

phase evident above S1.0, as shown by the diffraction results

to 6.3 ppm with an increase in silicon content. These changes

(Table 1). Therefore, the presence of this resonance results

are likely to be due to the local differences in the template-

from the AEI fraction within the intergrowth structure.



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framework interactions in the AEI and CHA cages found in the AEI, intergrowth and CHA components present within the different samples. S0.0 and S3.6 were calcined to investigate the differences between the

27

Al and

31

P NMR spectra of these

two calcined materials. For both nuclei, the spectra for S0.0 and S3.6 were identical (given in Supporting Information, Figure S2).

27

Al NMR showed an asymmetric resonance in

the four-coordinate region and a small resonance in the sixcoordinate region due to atmospheric water in the pores. Both calcined materials give a single

31

P NMR resonance, as

expected for calcined AlPO-18 and SAPO-34.24,32 In the calcined materials the phosphorus resonances overlap, thus it is important to use the as-synthesized spectra to monitor changes between SAPO-34 and SAPO-18. Microscopy. Scanning electron microscopy was used to determine the morphology of the materials and the results are shown in Figure 6. It is immediately clear that there is a significant change in crystal size and morphology between the low silicon materials (S0.0-S1.0) and the high silicon materials (S1.7-S7.0). The low-silicon materials (Figure 6a-d) have square, plate-like morphology consistent with SAPO-18.35 When the amount of silicon was increased above 1.0% (Figure 6e-h) the crystal morphology became cube-like

Figure 6. Scanning electron microscopy images of a) S0.0; b) S0.5;

and highly faulted. The average crystal size reduced from 1-4

c) S0.7; d) S1.0; e) S1.7; f) S3.6; g) S4.8 and h) S7.0. Scale bar is 2 µm

µm to less than 1 µm. These changes in morphology are

on all images.

consistent with the calculated level of intergrowth given in Table 1, as significant changes in morphology were observed after the AEI phase reduced to zero and the level of intergrowth increased above 60%. The level of SAPO-18 in S1.0 was low (13.1%) but the crystals maintained the platelet morphology. It seems the presence of AEI structure influences the crystal morphology in these materials, even at low levels. When the intergrowth becomes dominant (S1.7 and above) the crystals tend towards a more cubic, SAPO-34 type morphology.

AFM was used to probe the surface of the materials in order to determine their crystal-growth mechanism and to identify the intergrowth. This is possible as the shape of the surface features and the step heights are generally characteristic of the framework structures. The surfaces of SAPO-18 (S0.5) display complex spiral growth patterns (Figure 7a). Interlaced terraces and triangular terraces were visible on all crystals (schematic given in Figure 7b). The step height across the terraces under the blue line on Figure 7a is 1.7 ± 0.1 nm (Figure S4a). This


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corresponds to the unit cell length in the c direction of SAPO-

spiral has a regular pattern and step height of 0.9 nm (± 0.1

18, indicating that the large facet of the platelets is the {001}

nm; one common repeat layer), indicative of the CHA

face. The unit cell contains two mirrored common repeat

structure. The crystals exhibit different spiral patterns

layers because of the alternation in orientation of the D6Rs

depending on which type of structure has grown. There were

(Figure 1b). The step height across the interlaced terraces is

some crystals that exhibited solely spirals of SAPO-18, but all

an individual layer; i.e. half the unit cell (Figure S4b). The

had some degree of faulting and macrosteps on the surface

terraces interlace along the [100] direction due to anisotropy

because of the intergrowth component, which was not

in the growth rates of each individual layer in the [110] and

observed in S0.5. Overgrowth of smaller crystals was also

[11 0] directions. Triangular-shaped terraces were observed

common; in many cases these did not move during scanning

due to formation of an additional terrace edge. Here, these

in contact mode, indicating a direct connection with the larger

features (interlaced and/or triangular terraces) are used as a

crystal underneath rather than random placement of

fingerprint for AEI structure in the materials.

individual crystals during sample preparation.

Figure 7. a) AFM vertical deflection image of S0.5 (cross section analysis given in the supporting information, Figure 4a-b); b) schematic of the terrace structure.

X-ray diffraction showed S0.7 and S1.0 were the only materials with all three phases present (Table 1) and the crystals had comparable crystal size and morphology to S0.5 (Figure 6b-d). S0.7 consists of approximately a third of each of the AEI, CHA and intergrowth phases. The crystal surfaces

Figure 8. a-b) AFM vertical deflection images of S0.7 crystals; c) magnified area shown by the box in (a); d) magnified area shown by the box in (b). Cross section analysis given in Figure S4c-d.

appear more faulted than in S0.5 with channels and macrosteps visible on the surfaces (Figure 8a,b). Figure 8c is a magnification of Figure 8a, which shows an AEI-type surface

Figure 9a shows an AFM image of a S1.0 crystal. The

with interlaced terraces clearly visible. The step height across

crystals show a block-type pattern on the surface due to

the line in Figure 8c (Figure S4c) was consistent with the

increase of the intergrowth component (Figure 9a). This

height found in SAPO-18 (S0.5) confirming the AEI structure.

block-like growth is particularly interesting in the crystal

The crystal shown in Figure 8d, however, shows an example

shown in Figure 9a because separate regions show

of a more uniform spiral with some adjacent fault lines. The

characteristics of the different framework structures within



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Page 10 of 19

one crystal. Macrosteps are visible in the lower left corner of the crystal (Figure 9a), which is a result of the increased amount of intergrowth phase in this material. Also apparent are some intergrowths rotated at an angle to the main crystal suggesting some twinning. Figure 9b shows the region highlighted blue on Figure 9a. This shows a SAPO-34 type of spiral, similar to the one seen in Figure 8d, with a typical spiral shape and no interlacing. The step height was found to be 1.0 nm (± 0.1 nm, Figure S4e), consistent with the CHA unit cell. The area highlighted green, Figure 9c, shows another example of AEI growth where interlaced terraces are visible with step heights consistent with the AEI structure (1.7 nm ± 0.1 nm, Figure S4f). Layer growth is partially visible in the top part of Figure 9c and shows a ragged, curved terrace with a 0.9 nm (± 0.1 nm) step height. This is likely to be a distinct region of intergrowth structure within the crystal, which has grown via a layer growth mechanism (see below).

Figure 9. a) AFM vertical deflection image of a S1.0 crystal; b) magnified area shown by the blue box on (a); c) magnified area of the green box on (a). Cross section analysis given in Figure S4e-f.


Page 11 of 19

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present system the former is not possible as the crystals grow

a)

from an opaque gel, however, it is possible to dissolve the

b)

crystals in a weakly acidic, pH = 5, solution of phosphoric acid. These conditions were chosen because: a) the conditions mimic the growth conditions whereby the starting pH was 4, rising to pH 9 by the end of the synthesis as phosphorous is incorporated into the lattice; b) pH 5 results in a dissolution

0.5 µm

0.5 µm

The in situ experimental result is shown in the movie in the

d)

c)

rate compatible with the timescale of the AFM experiment.

Supporting Information, where a multiple screw dislocation is clearly shown to retreat via the inverse of a classical growth pathway. Indeed as we have shown previously, by using in situ AFM of nanoporous materials from zeolites36-39 to zeotypes40-42 and to MOFs43-50, a classical growth process is

0.5 µm

0.5 µm

an important general mechanism for the growth of such

Figure 10. AFM vertical deflection images of selected typical

crystals and it seems that the same is true for the current

intergrowth crystals; a-b) S3.6; c) S7.0; d) S1.7.

system. There is an ongoing debate in the literature as to the importance in crystal growth mechanisms of, what are sometimes termed, non-classical crystal growth pathways

From S1.7 to S7.0, there was a change in crystal size and morphology as shown by SEM (Figure 6) due to the increase of the intergrowth component. This change was also observed by AFM where many crystals were highly faulted with substantial bulk intergrowths (Figure 10a). Macrosteps

whereby there is a transformation of colloidal-sized amorphous particles into crystalline nanoporous framework materials, as well as the formation of pre-formed units that dock and grow into the final crystal.51-53 However, we have no evidence of such processes in the current system.

and block-type growth are visible in Figure 10b,c; which is a common feature for the intergrowth materials that was also

DISCUSSION

observed on many of the crystals with lower silicon content (Figure 8a and Figure 9a). The most significant feature for the intergrowth materials (S1.7 – S7.0) was evidence of a layer growth mechanism only and no spiral growth. Figure 10d shows a crystal with multiple, irregular rounded terraces from multiple individual nucleation sites. Step heights on this crystal were 1 or 2 common repeat layers high. All the AFM images described thus far are ex situ images recorded on samples that are quenched at the end of the synthesis. In order to capture the full essence of the crystal growth pathway it is also useful to have in situ images recorded under conditions of growth or dissolution. In the

The systematic changes in phase composition of the materials were determined by X-ray diffraction. Fitting of the diffraction patterns was used to quantify the level of the different phases in the materials as shown in Table 1. Implicit to the modeling approach used, it is possible to analyze the phases in more detail by calculating the relative amount of CHA or AEI layers in the materials – that is in the three phases listed in Table 1. The proportion of each layer type was calculated by XRD (details given in Experimental section) and the fraction of the CHA layers as a function of silicon content in the materials is given in Figure 11a. The



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Page 12 of 19

materials with the lowest levels of silicon (less than 0.7%)

calculations from the X-ray diffraction data, both deriving the

have very low levels of CHA layers, but the proportion of

CHA and AEI components from the three phases present in

CHA layers rapidly increases within a short range of silicon

the samples. 13

content, from S0.5 – S1.7 (Figure 11a). This is due to the

C MAS-NMR of the structure-directing agent

increase in the level of intergrowth and SAPO-34 fraction

(TEAOH) in the pores was used to indirectly observe the

over this short range of silicon content. The relative

local changes in the structure. The spectra showed a subtle

proportion of AEI layers has an inverse relationship (not

change in peak position from 6.9 to 6.3 ppm with increase in

shown).

silicon content. Assuming the resonance at 6.9 ppm It was also evident from the MAS-NMR spectra that

corresponds to the SDA in AEI cages, and 6.3 ppm

there were systematic changes as the level of silicon

corresponds to SDA in CHA cages, the deconvolution of the

increased. Figure 5c shows the

31

P spectra of the materials.

AlPO-18 (S0.0) showed three resonances at -12, -29 and -30 32

13

C NMR spectra provides a means to calculate the ratio of

CHA component in each material. The result of the

ppm, which is consistent with the literature. The resonances

calculation of the CHA fraction from the 13C NMR is given in

at -12 and -30 ppm progressively reduced in intensity as the

Figure 11c. There is a higher uncertainty in the results due to

amount of silicon was increased, but a small resonance at -12

the broad nature of the peaks in the NMR, but the overall

ppm was still present up to 7.0% silicon content. Xu et al.

trend is consistent with the XRD and the

reported a similar result where two SAPO-34 samples had a

calculations.

small resonance at -12 ppm, which they attributed to a secondary

SAPO-18

phase

or

the

presence

of

an

26

intergrowth. Pure-phase SAPO-34 shows a single resonance at -29 ppm. Therefore, the resonances at -12 and -30 ppm can be used as a measure of the AEI component in the materials, whereas the peak at -29 ppm is a result of overlapping CHA and AEI peaks. It was possible to deconvolute the peaks to determine the relative contributions from each framework type. We performed a constrained peak fit to calculate the amount of each structure type in the materials. Since, the AEI resonances occur in a 1:1:1 ratio, the calculated peak area of the -12 ppm resonance is used to define the area of the other AEI peaks at -29 and -30 ppm, with the remainder of the area of the -29 ppm peak being assigned to the CHA fraction. Examples of the peak fitting are given as Figure S3. The correlation of the calculated relative areas, representing the CHA and AEI components, against silicon loading is shown in Figure 11b. AlPO-18 (S0.0) has no CHA component but the proportion increases significantly over a short range of silicon content. These results give an excellent correlation with the


31

P NMR

Page 13 of 19

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NMR to characterize the intergrowth and it confirms the accuracy and reliability of the XRD analysis. With combination of both XRD and NMR we confirm that the NMR identifies the CHA and AEI components within the intergrowth phase, and does not simply indicate a coexistence of CHA and AEI in a mixed phase system. Further, the correlation between NMR and XRD strongly suggests that the changes in the

27

Al and

31

P NMR spectra are primarily

caused by the intergrowth composition rather than by local chemical

differences

caused

by

changes

in

silicon

concentration. Why the silicon content effects these structural changes is a matter of conjecture but it could be that the silicon prefers to reside in a CHA cage rather than an AEI cage. Direct observation of the materials is crucial to fully understand the formation of these intergrowths. AFM showed that AEI crystals grew via a complex spiral growth mechanism. Areas of pure CHA also grew via a spiral growth mechanism but the spiral was uniform in shape with a Burgers vector of 0.9 nm. Conversely, the intergrowth crystals grew via a layer-by-layer growth mechanism. In the case of S1.0 (Figure 9) all three phases were visible on one crystal, indicating that these materials are not simple physical mixtures of three distinct phases. There is no indication of overgrowth of the intergrowth phase on the CHA or AEI spirals. There are examples in other systems of screw dislocations promoting polytypism in specific circumstances.54 However, there is also evidence for the desired polytype being maintained by crystal Figure 11. Proportion of CHA component, against silicon loading, calculated by a) XRD, b)

31

P NMR and c)

13

C NMR.

Points are connected to guide the eye.

growth on an existing spiral growth hillock.55 In the case of the CHA/AEI system, the AFM evidence strongly suggests that the spiral growth mechanism promotes pure-phase end-

The results in Figure 11 show a self-consistent agreement with respect to the analysis of the relative fractions of the CHA/AEI components probed in three different ways. There is a good correlation between the techniques, which demonstrates the potential effectiveness of using

31

P or

13

C

member growth and discourages intergrowth formation. Therefore, the presence of spirals will not allow intergrowth formation on those parts of the crystal. Figure 12 shows the cage structure of the CHA and AEI spirals. The structure and Burgers vector of each dislocation type does not allow



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Page 14 of 19

Figure 12. Cage structure of the CHA (orange) and AEI (blue) spiral growth hillocks. Intergrowth of the two structures via spiral growth is not permitted, thus a boundary is formed between each spiral type.

intergrowth of the other structure on the spiral. The intergrowth therefore proceeds exclusively via the layer growth mechanism observed in Figure 10d. In the layer-bylayer growth mechanism the switching of the layers is completely random (Figure 13). At low levels of intergrowth (i.e. in S0.7 and S1.0) the spiral growth mechanism still dominates but there is competition between growth of CHA, AEI and the intergrowth phases. Consequently, a block-like growth occurs on the platelets, giving macrosteps and channels along the surface. In high levels of intergrowth (60% and above), the layer growth mechanism dominates thus many crystals were highly faulted leading to macroscopic levels of intergrowth

Figure 13. Random layer structure of the CHA/AEI intergrowth. The line represents the boundary that is formed when 2 different layer types cannot grow laterally.

(Figure 10a). Because of the competition between growth possibilities it is also evident that within one sample there is a degree of heterogeneity.

Block-like growth of different regions on the surface occurred due to interruption in growth across the crystal. In the case of spiral growth, the framework type is defined by the Burgers vector of the screw dislocation, i.e. 1.8 nm for AEI or 0.9 nm for CHA. When a spiral growth hillock grows


Page 15 of 19

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out from a particular type of screw dislocation (CHA or AEI), the terraces cannot coalesce with terraces from another spiral type and hence a boundary is formed (Figure 12). In the case of layer growth, the switching of the common repeat layers is random to give a mixture of CHA and AEI layers within the intergrowth crystal (Figure 13). If two separate nucleation sites of the same cage type occur on a single terrace, then the step growth perpendicular to the surface is allowed and the terraces coalesce. However if a CHA and an AEI cage both nucleate on a single terrace then the layers cannot combine as they are growing laterally and a boundary will form, although subsequent overgrowth of a complete layer is then possible. This block-like growth gives the materials higher-level hierarchical structure that cannot be observed by diffraction and spectroscopic methods.

Figure 14. Phase diagram based on the phase fractions calculated by DISCUS. The lines have been smoothed and the data points removed for simplicity. Each crystal growth type is indicated for the different phases present in the materials.

Both the spiral growth and layer growth mechanisms are in competition in this system, where spiral growth inhibits intergrowth formation. This could have implications for

CONCLUSIONS

control of the intergrowth because spiral growth and layer growth are affected by supersaturation: spiral growth

We have synthesized a well-defined series of

dominates at low supersaturation whereas layer growth is

CHA/AEI

typical of higher supersaturation. Therefore, it is conceivable

mechanism at different levels of intergrowth. The AFM

that layer growth, and thus the intergrowth component, could

observations are hinged on the high-resolution X-ray

be induced or reduced by alteration of the supersaturation

diffraction data paired with sophisticated modeling, which

levels during synthesis.

allowed detailed analysis of the phase composition and level

All three types of crystal growth are summarized in

materials

and

probed

the

crystal-growth

of intergrowth. The phase composition over the series was 31

P and

13

Figure 14 on the CHA, AEI and intergrowth phase diagram.

examined by XRD and

C MAS-NMR. A strong

At low levels of silicon either AEI or CHA spirals are

correlation was found between all three sets of data,

formed. At the higher levels of silicon, layer growth

demonstrating the consistency and accuracy of both the long-

dominated and the spirals are no longer observed. All the

range (XRD) and short-range (NMR) techniques. This

techniques employed are important for the deconstruction of

information could then be matched with the microscopy data,

the intergrowth structure but the AFM observations are key to

revealing the nature of the crystal growth. Pure-phase CHA or

revealing the growth of the CHA/AEI system. These

AEI regions formed via a spiral growth mechanism and both

observations can likely be applied to other systems with a

spiral types are non-equivalent and unable to intergrow.

propensity for screw dislocations.

Therefore, intergrowth could only occur by layer switching in a layer-by-layer growth mechanism. Highly intergrown crystals display layer growth with no evidence of spiral growth. Lower levels of intergrowth are observable as faults



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Page 16 of 19

across the crystal surface. Block-like growth occurs where

Norway, grant number 208325 and the synchrotron  and

different regions are not able to intergrow, resulting in

neutron travel grant, number 216625.

channels across the surface, giving rise to a higher-level hierarchical structure. SAPO-18 and SAPO-34 regions could

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Nanoporous Intergrowths: How Crystal Growth ... - ACS Publications

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