Effect of Organic Templates on the Kinetics and ... - ACS Publications

data was then analyzed using the analysis suite available from. Daresbury laboratories .... and the evolution of the area under the single (0 2 1) ref...
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J. Phys. Chem. C 2007, 111, 16951-16961

16951

Effect of Organic Templates on the Kinetics and Crystallization of Microporous Metal-Substituted Aluminophosphates Matthew G. O’Brien,*,†,⊥ Manuel Sanchez-Sanchez,†,‡ Andrew M. Beale,§ Dewi W. Lewis,| Gopinathan Sankar,† and C. Richard A. Catlow†,| DaVy-Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London, W1S 4BS, U.K., Department of Chemical and EnVironmental Engineering, ESCET, UniVersidad Rey Juan Carlos, C/Tulipan, s/n, 28933 Motoles, Madrid, Spain, Inorganic Chemistry and Catalysis, Department of Chemistry, Utrecht UniVersity, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands, and Christopher Ingold Laboratories, Department of Chemistry, UniVersity College London, 20 Gordon Street, London, WC1H 0AJ, U.K. ReceiVed: June 28, 2007; In Final Form: August 29, 2007

The formation of the microporous cobalt-containing aluminophosphate CoAPO-36 (ATS framework), synthesized using three different organic templates, tripropylamine (TPA), N,N-diisopropylisobutylamine (DPBA), and N-ethyldicyclohexylamine (ECHA), has been examined using in situ energy-dispersive X-ray diffraction (EDXRD). These measurements revealed that the type of template can significantly affect the rate of formation, the order being DPBA > ECHA > TPA. This observation is further supported by the measurement of induction times and activation energies (using the Arrhenius equation), which also follow the same trends. Further ex situ crystallographic studies of the as-synthesized materials have then shown that the template can also alter the size and symmetry of the unit cell of these materials. By combining these results with computational calculations on the size of each template and its packing within a representative section of the framework, we have shown that the bulky nature of the ECHA template is responsible for the changes in the unit cell, while the interaction energies between the template and the framework at different packing levels rationalize the EDXRD results, showing that the system with the most stable interaction energy will form more rapidly.

Introduction Aluminophosphates (AlPOs)1,2 constitute part of a family of microporous materials defined as containing pores of less than 2 nm in diameter,3 which confers on them a number of potential applications, including molecular sieving and shape-selective catalysis.4,5 Despite their catalytic potential, to date little industrial-scale usage of aluminophosphates has been realized. However, this situation is likely to change in the near future as these materials are active for the conversion of methanol to olefins and light hydrocarbons, which offers an alternative to a reliance on oil feedstocks for such products.6-8 The rational design of AlPO-based microporous materials has yet to be achieved, due at least in part to the fact that their formation mechanism is still a matter of considerable debate. A large number of studies have focused on the catalytic activity9-15 of these systems and on the effect that variables such as synthesis temperature16,17 and time,17 the nature of the template18 and its interaction with the forming framework,19-21 and the presence of substituted metals22-24 within the gel have on the final crystalline phase(s). However, very little work has focused on how the type of template can affect the underlying kinetics of formation of a particular system and, consequently, its exact crystal structure. A greater understanding of these * Corresponding author. E-mail: [email protected]. † The Royal Institution of Great Britain. ‡ Universidad Rey Juan Carlos. § Utrecht University. | University College London. ⊥ Current address: Department of Inorganic Chemistry and Catalysis, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands.

factors will improve our ability to predict which microporous structure a particular template will form and the energy required for this formation. This will become increasingly important as the industrial application of these materials becomes more widespread, allowing routes to new microporous aluminophosphate materials and offering lower energy (and therefore lower cost) routes for the production of existing materials. In this work, we have examined the effect of the template on the rate of formation of cobalt-substituted AlPO-36 (CoAPO36). AlPO-36 with the ATS structural topology1,2 is characterized by unidirectional elliptical (6.5 × 7.5 Å) channels with 12-ring apertures. The presence of staggered annular side pockets inside the channels enlarges the diameter to 10.1 × 9.2 Å,25,26 giving rise to high sorption capacities compared to similar AlPO materials. Additionally, metal ion substitution into aluminum framework sites can create strong Brønsted acid sites.27 These features give hetroatom-doped AlPO-36 (MeAPO36) potential as a catalyst in areas such as n-butane cracking28 and aromatic formation.29 Of particular note is the potential of CoAPO-36 as a redox catalyst in the industrially important processes of K-A oil (cyclohexanone and cyclohexanol) and adipic acid production, which is currently manufactured using environmentally and economically costly homogeneous catalysis.30 Here cyclohexane is initially oxidized to K-A oil and is then converted to adipic acid over longer periods of time.11 MeAPO-36 is generally synthesized following methods based on those originally adopted by Flanigen et al.,31 and a typical example is given by Machado et al.32 Until recently the formation of a pure MeAPO-36 phase had always proved difficult due to the competitive formation of AFI structured

10.1021/jp0750351 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/23/2007

16952 J. Phys. Chem. C, Vol. 111, No. 45, 2007 materials.27,33,34 Pure materials could only be prepared using tripropylamine (TPA) as the template and only under a carefully controlled two-step hydrothermal treatment, taking almost 100 h. While this method results in high-purity products in high yields, the long complex synthesis procedure made the in situ study of the formation of these systems difficult, expensive, and time-consuming. However, recent advances have demonstrated that, with careful selection of the right starting materials and synthesis conditions, it is possible to prepare phase pure CoAPO-36 using the conventional TPA template, in a fast onestep hydrothermal process at constant temperature. Additionally, after two decades in which TPA was the only template known to produce the ATS framework of AlPO-36, the templates N,Ndiisopropylisobutylamine (DPBA) and N-ethyldicyclohexylamine (ECHA) were also found to direct the synthesis of CoAPO-36.35 These advances therefore offer a unique opportunity to study the kinetics of crystallization of AlPO-36 using in situ methods and examine the effect of different templates on this process. We will also show how variations in the template, as well as affecting the rate of formation of the materials, can subtly alter the crystal structure of the AlPO, which can partially explain the observed rates. Computational studies have then thrown light onto these effects and demonstrated that each template has different interaction energies with the AlPO framework, which again can be related to the rate of crystal formation, while the size and occupied volume can explain the changes in the crystal structure. Experimental Section Synthesis of CoAPO-36. CoAPO-36 phases were prepared in the laboratory using a hydrothermal treatment with the composition 1.0 P:0.9 Al:0.1 Co:0.8-0.9 Z:10 H2O, where Z is either TPA (99%, Aldrich), DPBA (98%, Aldrich), or ECHA (98%, Aldrich) and the ratio was varied slightly to allow for formation of pure CoAPO-36. In a typical synthesis procedure H3PO4 (85% in water, Aldrich) was dissolved in H2O (doubly distilled) with mechanical stirring for about 5 min. (CH3COO)2Co‚4H2O (98%, Sigma-Aldrich) was then added slowly to the solution under continual stirring. Upon dissolution, Al(OH)3‚xH2O (Sigma) was slowly added to the solution under vigorous stirring until a homogeneous gel was achieved (ca. 5-10 min). Finally, the desired template was added to the gel, initially dropwise until solidification occurred and then rapidly with vigorous stirring by hand. After a few seconds the gel became more fluid and was then stirred mechanically for 1 h, until it became a viscous homogeneous gel. Gels were then placed in Teflon (PTFE) lined stainless steel autoclaves and heated for 5-20 h at 160 °C. Postsynthesis products were filtered, washed with distilled water, and dried at 100 °C. In situ experiments were prepared in an identical manner, but the gels were added to specially machined autoclaves for energydispersive X-ray diffraction (EDXRD) studies detailed below. Calcined samples were prepared by heating the as-prepared laboratory materials in an air flow furnace at 1 °C/min to 550 °C for 5 h. Characterization. XRD patterns were recorded using a Bruker D4 diffractometer in the Bragg-Brentano orientation using a rotating flat plate in order to reduce any preferred orientation. The X-rays were generated from a copper source emitting Cu KR1 (1.540 598 Å) with a secondary monochromator to remove any Cu KR2 and Cu Kβ radiation and using a step size of 0.007° 2θ. The Fullprof suite of programs36 was used to perform Le Bail fits on the XRDs, and initial fitting

O’Brien et al. parameters were based on those given previously by Smith et al.25 Scanning electron microscopy (SEM) images were taken using a JEOL JSM-6301F scanning microprobe and energydispersive X-ray analysis (EDX) was performed using a Hitachi EDX 5570 fitted with an Oxford Instruments INCA analysis suite for data processing. Extended X-ray adsorption fine structure (EXAFS) measurements were carried out at station 9.3 of the SRS at Daresbury laboratories using a Si(220) double crystal monochromator in transmission mode. Samples consisted of 40 mg of laboratory prepared CoAPOs pressed into 13 mm pellets and mounted for measurement using station-based mounting equipment.37 The data was then analyzed using the analysis suite available from Daresbury laboratories, namely EXCALIB, EXSPLINE, and EXCURV98. Energy-dispersive X-ray diffraction (EDXRD) was performed using the high flux white beam available at station 16.4 of the SRS. A three-element detector, previously utilized for monitoring hydrothermal crystallization, was used for these experiments,18,38 held at fixed 2θ angles of 1.3°, 4.18°, and 7.08°. The sample environments consisted of stainless steel autoclaves with a PTFE liner (∼40 mL sample capacity) specifically designed with thin walls to maximize the radiation flux passing through the sample.39,40 The raw data files were smoothed, calibrated, and outputted in standard two-column ASCII format using the XRD explorer software program available on the station, and all further conversions of data to appropriate formats were performed using in-house developed macros. Peak areas were integrated using the Gaussian curve fitting routine available in the XFIT peak profiling software.41 In a typical experiment an autoclave containing the appropriate gel mixture was introduced into an in situ oven preheated to the desired temperature. The time between introduction of the sample to heat and the beginning of measurements was 2 min, and a data collection time of 1 min/scan was used for all experiments. Computational Details Molecular Mechanics. The molecular mechanics (MM) calculations performed are similar in methodology to those previously applied to the study of template-zeolite interactions.42-44 Template molecules were built in Materials Studio45 and their optimized geometries were calculated with the Discover code46 using the ab initio derived force field cff91_czeo, with parameters as detailed by Hagler et al.47 and Hill and Sauer.48 After the energy minimization the molecules were then inserted into a (1 1 4) supercell (i.e., four unit cells) of the ATS framework and the system was again energy minimized, but without relaxation of the framework. Due to the relatively small and rigid nature of the templates and the small pore space of the ATS framework, the likely number of orientations within the structure is limited. Therefore, each template molecule was inserted into the supercell either parallel to the YZ, YX, or ZX orientation or at 45° between two of the planes, YX(45)ZX and YZ(45)ZX (Supporting Information figure S1). Additional molecules were then inserted in identical orientations to pack the channel. All comparative energies in this work are quoted as the interaction energy (Einter) defined as

Einter ) Ehost - Efree where Ehost is the total energy of the framework/template combination and Efree is the energy of an isolated gas-phase template molecule.

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Figure 1. XRD patterns of as-synthesized CoAPO-36 materials synthesized using TPA (a), ECHA (b), and DPBA (c). The tick marks represent the peak positions of standard ATS;22 slight deviations from these are observed due to template effects.

Figure 2. Cobalt K-edge EXAFS data (left) and their corresponding real space Fourier transforms (right) for TPA (a), ECHA (b), and DPBA (c), with solid lines representing the actual data and dashed lines representing the fits.

A number of constraints were applied to the systems investigated, necessary to allow the calculations to be performed with the computational resources available, all of which have been previously applied to zeolitic systems. These include, as noted above, the use of a rigid framework for the template, the use of a purely siliceous framework to represent the AlPO system,49 the omission of electrostatics in the system, and the omission of substituted cobalt from the framework. Such approaches have been previously used to study the templateframework interaction,42,49-52 reproducing experimental template geometries42,44 and providing a useful correlation between these interactions and crystallization times in other microporous systems.43 (Note: these constraints and their validity are discussed in more detail in the Supporting Information.) Molecular Size Measurements. For comparison, the size of the template molecules used in these experiments was measured using two techniques. In the first method the difference between the nuclear positions of atoms at the extremities of the template molecules were measured in three dimensions representing the maximum length, width, and height (effectively corresponding to X, Y, and Z dimensions) (Supporting Information figure S2). The van der Waals volumes of the molecules were then calculated using Materials Studio.45 Second, we consider the principal axis of inertia for each template as a measure of the template size. These axes are

Figure 3. Stack plot resulting from the accumulation of many 1 min scans (a) demonstrating the growth of several reflections attributed to CoAPO-36 as a function of time. (b) Resulting S-shape crystallization curve taken by measuring the area under the (0 2 1) reflection. (Synthesis using the TPA template at 150 °C.)

derived by considering the total moment of inertia I of a polyatomic molecule:

I)

∑i miri2

Here, the mass m of each atom i is multiplied by the square of ri, which is the perpendicular distance of the atom from the axis of rotation of the molecule.53 The principal axes RX, RY, and RZ are then inversely proportionally to the corresponding orthogonal moments IX, IY, and IZ and are scaled to describe an inertial ellipsoid around the molecule. Boyett et al.50,51 have previously demonstrated that this provides a quantitative description of the structure of organic templates, and both methods allow for good comparison between each template. Results and Discussion Initial Phase Analysis. Prior to performing in situ experiments, CoAPO-36 samples synthesized with the different templates were compared for phase purity and the extent of cobalt substitution. The dried, filtered products were subjected to analysis by powder XRD and were found to be consistent with the AlPO-36 ATS structural framework (see Figure 1).25 Some deviation from the standard ATS peak positions are noted in the sample synthesized with the ECHA template, particularly in the 20-25° 2θ region of the pattern, which we attribute to the effects of the template on the framework, as discussed in detail below. Additionally, the homogeneous morphology of

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TABLE 1: Resulting Coordination Number (N), Cobalt-Oxygen Distances (R), Debye-Waller Factors (σ2), and R-Factors for Each As-Prepared CoAPO-36 Sample Obtained from the EXAFS Fits Shown in Figure 2a

a

sample (template)

N

Me-O distance R (Å)

2σ2 (Å2)

R-factor (%)

CoAlPO-36 (TPA) CoAlPO-36 (ECHA) CoAlPO-36 (DPBA)

3.7 (0.27) 4.1 (0.26) 3.9 (0.22)

1.939 (0.006) 1.938 (0.006) 1.939 (0.005)

0.011 (0.002) 0.011 (0.002) 0.013 (0.002)

22.01 21.80 21.63

Numbers in parentheses represent deviations for least-squares fits.

particles detected in the SEM images (see results) suggests that no additional phases are present. Extensive substitution of tetrahedral cobalt(II) into tetrahedral aluminum(III) framework sites within the AlPO framework was initially indicated by the homogeneous deep blue color of the samples.54 Further evidence was obtained from EDX which indicates that, of the combined total of cobalt and aluminum atoms in the sample, 10.9%, 10.8%, and 11% of the atoms in the TPA, ECHA, and DPBA samples respectively are cobalt, indicating that the final products have a composition similar to that of the original synthesis gels. To obtain further knowledge of the local environment of this cobalt, XAS measurements were performed on the as-prepared samples. The resulting EXAFS data and the associated realspace Fourier transforms (FTs) for each sample are shown in Figure 2 and the calculated coordination number (N), cobaltoxygen distance (R), Debye-Waller factor (σ2), and quality of fit (R-factor) are given in Table 1. In each sample, the cobalt is coordinated to approximately four neighbors, as would be expected in a tetrahedral environment. The average Co-O bond distance of 1.94 ( 0.02 Å is also similar to the Co-O bond distance of the well-characterized spinel structure, CoAl2O4 (in which high-spin Co(II) mainly occupies the A-site55) and to those in other similar cobalt-substituted AlPO-n materials.56-58 The Debye-Waller factors are also similar to each other (σ2 ) 0.012 ( 0.001 Å2), suggesting that there is no significant static disorder around the cobalt environments. These results demonstrate that CoAPO-36 with the ATS structural framework has been successfully synthesized using each template and that in each case the cobalt atoms have substituted successfully into aluminum tetrahedral sites. In Situ Kinetic Measurements. To obtain detailed information on the kinetics of crystallization of CoAPO-36 synthesized using each of the templates, a detailed in situ EDXRD study was undertaken at various temperatures. Similar studies have been undertaken on a variety of zeolitic16,59-61 and nonporous materials,62-64 and EDXRD is of particular use for in situ studies as the polychromatic “white” beam allows the use of sample environments that are as close to “real” as possible (such as stainless steel autoclaves).65,66 A typical plot from the middle and lower angle detectors of the EDXRD setup revealed the growth of pure CoAPO-36 phases using this experimental environment (Supporting Information figure S3). An example of the time-resolved data from the middle detector (displayed as a three-dimensional stack plot) and the evolution of the area under the single (0 2 1) reflection for TPA-synthesized CoAPO-36 at 150 °C s is given in Figure 3. A characteristic “S-shape” growth curve is observed with an initial induction/nucleation period and subsequent growth which terminates after a period of time. Comparison of the area under a number of reflections from a single experiment revealed growth curves which are generally superimposable over the duration of the experiment, indicating that growth is isotropic on all crystallographic planes; therefore, all peak areas should be representative of the growth mechanism occurring (Supporting Information figure S4). For subsequent analysis the (0 2 1)

TABLE 2: Induction Time, Exponent (n), and Rate of Reaction (k) for the Crystallization of CoAPO-36 from Gels with Different Templates at a Number of Temperaturesa SDA TPA

ECHA DPBA

a

temp (°C)

induction time (min)

n

k (min-1)

130 140 150 160 140 150 160 130 140 150 160

65 36 27 26 32 23 20 27 22 16 14

1.7 1.9 1.7 2.0 2.1 1.9 1.9 1.8 1.5 1.5 2.4

0.034 0.054 0.137 0.217 0.112 0.233 0.263 0.158 0.215 0.265 0.293

Errors on n and k are estimated to be (8%.

reflection was measured, as its distance from other reflections was significant enough to allow a simple Gaussian peak fitting procedure to be employed. For all subsequent analysis, it is convenient to calculate the extent of reaction (Rhkl) by considering the integrated peak intensity of a particular normalized Bragg reflection Ihkl at time (t) compared to the intensity of the same reflection at the completion of the reaction (t∞):

Rhkl(t) )

Ihkl(t) Ihkl(t∞)

Due to the dynamics of the reaction, the scattered intensity can vary during an experimental run, making calculation of the end point (R(t) ) 1) difficult. Therefore, the end point of a reaction was calculated using log I vs time graphs and a statistical analysis (Supporting Information). This fitting procedure was performed on all data sets, and Figure 4a displays results for the crystallization of CoAPO-36 from a gel containing TPA at different temperatures, whereas Figure 4b compares crystallization curves for samples prepared with each different template at a given temperature (160 °C). An increase in temperature leads, as expected, to a reduction in the induction period and a more rapid rate of crystallization (as seen from the steeper gradient of the growth curve). More interesting, however, is the behavior of systems crystallized with different templates at the same temperature. Here, the type of template has a clear and substantial effect on both the induction time and crystallization rate, with the DPBA system exhibiting a considerably reduced induction time and increased rate of reaction, compared to ECHA. TPA, on the other hand, shows extended induction times and a reduced rate of reaction compared to ECHA. By performing a systematic study on each of these systems at different temperatures, it was possible to obtain information on the kinetics of crystallization. The kinetic model proposed by Avarami67-69 and Erofe’ev70 was used for this analysis, with the overall course of the reaction being separated into nucleation and growth stages. This model has been widely used for analyzing a variety of solid-state processes such as phase

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Figure 4. Crystallization curves of CoAPO-36 using TPA (a) at varying temperatures and (b) using different templates at 160 °C.

transformations71 as well as crystallizations in zeolitic16,59-61 and other nonporous materials62,72 and is represented by

R ) 1 - exp[(-kt)n] Here, the extent of the reaction (R) is related to the rate constant (k) and the exponent (n) at a particular temperature (t). The exponent is known to be related to the mechanism of both nucleation and growth, and Hulbert73 has analyzed a number of ideal reaction mechanisms, concluding that n contains information on the dimensionality of growth of the crystallization process, with typical values ranging from n ) 0.5 to n ) 4. The most straightforward way to extract information from the Avarami-Erofe’ev equation is by performing a SharpHancock74 analysis of the data, involving a double logarithm:

ln[-ln(1 - R)] ) n ln(t) + n ln k A plot of ln[-ln(1 - R)] vs ln(t) should then yield a straight line, giving both n from the gradient of the slope and k from the x-axis intercept. Such an analysis is generally performed when the extent of reaction has values of 0.15 < R < 0.8, as deviations during the very early and late parts of crystallization can affect the results.62,75 The resulting Sharp-Hancock plots for each CoAPO-36 system are given in Figure 5, and the derived rates of reaction and Avarami exponents are given in Table 2. The results confirm the observations taken from the initial data sets with the induction times in the order TPA > ECHA > DPBA and the rates of reaction in the order DPBA > ECHA > TPA. The exponents for each of these systems are found to have values generally between 1.5 and 2, indicating that a one-dimensional phase boundary controlled growth mechanism with a decreasing nucleation rate is occurring.73 While these exponent values are only indicative of the growth mechanism, similar values have recently been reported for the formation of AlPO-5 (AFI framework), which, like the ATS framework, is also a one-

Figure 5. Sharp-Hancock plots derived from crystallization of CoAPO-36 using the templates TPA (a), ECHA (b), and DPBA (c).

dimensional system.76,77 Additionally, an examination of SEM images of the final products from these reactions (Figure 6) revealed the presence of a one-dimensional rodlike crystal morphology, in come cases aggregated into three-dimensional spheres, similar to that reported previously for the ATS structure.24,27,78 This aggregation of some samples suggests that secondary aggregation of the rods may be occurring during some of the crystallizations, which may then explain why the exponent has values that are slightly greater than 2 for some experiments (as for three-dimensional growth n is 1.5-2.5). However, these values are often difficult to determine reliably, and can be significantly affected by outliers in the data. The Arrhenius expression, obtained by plotting ln(k) against 1/T (Figure 7), may then be used to calculate the activation energy (Ea) for each system. The derived values (Table 3) are similar to those previously reported for other hydrothermal syntheses16,59,79,80 and support the results from the SharpHancock analysis, with the DPBA system having the lowest apparent activation energy (30 ( 2 kJ mol-1), while the value for ECHA is more than double that (64.0 ( 5 kJ mol-1). TPA has by far the highest apparent activation energy (93 ( 7 kJ mol-1), being approximately 3 times higher than that of DPBA. From these results it is clear that the type of template used to synthesize a particular framework can significantly alter the rate at which the framework is formedsa result which may have important consequences for any industrial applications of these materials, as the development of lower energy, more environmentally friendly synthesis routes would be desirable. However, in order to use these variations in the rates of crystal growth, it

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Figure 6. SEM images, at increasing magnification (top to bottom), of CoAPO-36 synthesized using the templates TPA (left), ECHA (middle), and DPBA (right). The spherical nature of the ECHA and DPBA particles can be clearly seen, while the rodlike structure of some crystals is also clearly evident.

Figure 7. Arrhenius plots of CoAPO-36 synthesized using the templates DPBA (a), ECHA (b), and TPA (c).

TABLE 3: Apparent Activation Energies (Ea) Derived from Arrhenius Plots of CoAPO-36 Crystallization from Gels Containing Different Templatesa

a

SDA

Ea (kJ mol-1)

TPA ECHA DPBA

93 64 30

Percentage error is estimated to be (7%.

is important to understand their causes. To this end a more detailed examination of the slight variations in the XRD patterns measured above was performed. Crystallographic Studies. As noted in the initial analysis, some slight deviations from the ideal ATS structure were seen in CoAPO-36 synthesized using the ECHA template. In particular, a close examination of the 20-25° 2θ range reveals that while both DPBA and TPA have two nonequivalent peaks

Figure 8. The range 20-25° 2θ for each as-synthesized CoAPO-36 sample: TPA (a), ECHA (b), and DPBA (c). The arrows mark the (1 3 1) and (1 3 -1) peaks; in the case of ECHA only a single (1 3 1) peak is present.

identified as (1 3 1) and (1 3 -1), ECHA has only a single (1 3 1) peak with a much higher intensity (Figure 8). The ATS framework has previously been identified as existing in a number of symmetries, depending on factors such as the substitution of metal into the framework. In its highest symmetry (Cmcm),81 the (1 3 1) and (1 3 -1) peaks are known to be equivalent. However, in some metal-substituted systems, a variation in the unit cell parameter angles away from 90° can result in both a triclinic C1 symmetry26 and a monoclinic C2/c symmetry.25 In both cases, these lower symmetry systems lead to a splitting of the (1 3 1) and (1 3 -1) peaks. Smith et al.25 have also noted that, in the high-symmetry system, the coalesced peak intensity is considerably higher than the two in-equivalent peak intensities. From this information, it is proposed that in these as-synthesized systems, the ECHA-synthesized CoAPO36 has the higher symmetry (Cmcm), while the DPBA-

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Figure 9. Le Bail fits for CoAPO-36 synthesized using TPA (a), ECHA (b), and DPBA (c) fitted with the C2/c space group and the ECHA system fitted with the Cmcm space group (d).

TABLE 4: Unit Cell Parameters for Each As-Synthesized CoAPO-36 Systema

TABLE 5: Unit Cell Parameters for the Le Bail Fits of Each CoAPO-36 Sample after Calcination

parameter

TPA

DPBA

ECHA

ECHA

parameter

TPA

ECHA

DPBA

space group a (Å) b (Å) c (Å) volume (Å3) β (deg) (R ) γ ) 90°) Rp Rwp Rexp DW-exp

C2/c 13.0875(7) 21.638(1) 5.1647(2) 1460.8(1) 92.848(6)

C2/c 13.1733(5) 21.501(1) 5.1445(2) 1456.1 (1) 92.176(4)

C2/c 13.1473(5) 21.605(1) 5.2148(5) 1481.2 (1) 90.141(5)

Cmcm 13.1779(7) 21.632(1) 5.2279(3) 1491.3(1) 90b

4.90 6.62 2.91 1.9421

6.11 7.92 4.41 1.9413

5.98 7.78 3.20 1.9419

5.73 7.61 3.11 1.9254

space group a (Å) b (Å) c (Å) volume (Å3) β (deg) (R, γ ) 90°) Rp Rwp Rexp DW-exp

C2/c 13.167(3) 21.507 (4) 5.159 (1) 1458.9(5) 93.01(2) 6.15 8.41 3.25 1.9291

C2/c 13.127(3) 21.421(5) 5.158(1) 1448.2(5) 93.10(2) 8.53 12.3 3.2 1.9294

C2/c 13.109(2) 21.356(3) 5.164(1) 1442.6(5) 93.68(3) 7.60 10.7 3.16 1.9314

a The ECHA system has been fitted using both the C2/c and Cmcm space groups for comparison. b Note that for the Cmcm system β is not calculated, as R ) β ) γ ) 90°.

synthesized CoAPO-36 and TPA-synthesized CoAPO-36 have the lower symmetry (C2/c). To confirm the above hypothesis, Le Bail fitting procedures were performed on each diffraction pattern. Figure 9 displays the results, while Table 4 lists the final minimized parameters for each system. In every case we achieved reliability factors that are reasonable for a Le Bail fitting procedure and the results confirm the initial observations.82 Both the DPBA- and TPAsynthesized samples have the C2/c space group with a β-angle with significant deviation from 90° at 92.18° and 92.84°, respectively. The ECHA system was fitted in both the C2/c symmetry and the higher Cmcm symmetry. As expected from the absence of the (1 3 -1) reflection, the β-angle is almost

90° (90.14°) when fitted in the C2/c symmetry and fitting in the higher symmetry system resulted in slightly improved reliability factors. Comparisons of the other unit cell parameters show that samples containing both DPBA and TPA have similar unit cell volumes of 1456.1 and 1460.8 cm3 respectively, while the ECHA-containing sample has undergone a significant expansion to 1491.3 cm3. This effect is partly due to the shift in the β-angle toward 90° and also the increase in the unit cell along the c parameter, which increases from 5.144 Å (DPBA) and 5.165 Å (TPA) to 5.224 Å in the case of ECHA. These experiments indicate that synthesis of CoAPO-36 using ECHA as the template results in a noticeable difference in the framework, with the unit cell being both expanded and having a higher symmetry compared to the other systems. Smith et al.25 have previously suggested that, in calcined systems, the

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Figure 11. Plot of computational calculated interaction energies of each template-synthesized AlPO compared with experimentally derived activation energies (squares) and rates of reaction at 160 °C (circles).

Figure 10. Interaction energies for (a) TPA, (b) ECHA, and (c) DPBA molecules within the supercell of the SDA framework in each orientation at different concentrations within the supercell.

deviation from the higher symmetry Cmcm framework can be caused by a temperature-induced “crinkling” of the framework. It is therefore reasonable to consider that, in these as-synthesized samples, the ECHA template has the ability to expand the unit cell and “uncrinkle” the framework, an ability that is not inherent in samples synthesized with the other templates. It is possible that this effect may be at least partly responsible for the variation in the rate of reaction noted in the EDXRD studies. By repeating the Le Bail fitting procedure for calcined samples, it was possible to demonstrate this “crinkling” effect (Supporting Information figure S6). Some collapse of the framework, leading to inferior fits, has occurred in each case. However, a comparison of each fit still indicates extensive “crinkling” on template removal, resulting in β-angles that are greater than those of the as-synthesized materials (Table 5). In particular, the β-angle for the ECHA-synthesized sample changes from 90° to >93°. Computational Results. From the crystallographic studies, it is clear that ECHA causes an expansion of the ATS unit cell, while there is no significant difference between the TPA- and DPBA-synthesized systems. One explanation for this may be a difference in the size of the template molecule. Utilizing the two measurement methods described in the Experimental Section, each template’s dimensions and volume were measured (Table 6). From these results it is apparent that while the actual values for each parameter vary depending on the method, the trends remain the same and are therefore comparable. ECHA is found to occupy a considerably larger volume (both van der Waals

and molecular) than DPBA and TPA. If we now consider that there is a framework charge on the CoAPO-36 framework due to Co(II) substitution for Al3+, then the template must be present in its cationic form to balance this charge. As described in the Supporting Information, under acidic conditions the neutral amines used in this work become protonated. Therefore, if the same number of each template is packed into a given volume of the framework to balance the charge, then the ECHA template will occupy the largest volume, resulting in a higher energy system. An expansion of the unit cell would probably relieve some of this strain and reduce the lattice energy, as observed crystallographically. While the increased size and resulting expansion of the unit cell can explain the reduction in the rate of crystallization of ECHA compared to DPBA, it does not explain the significant retardation in the synthesis of the TPA system. Indeed, due to its planar structure this template occupies a smaller volume than DPBA. However, by examining the effect of each template on the energy of the ATS system using molecular mechanics, further differences are observed. In each case we attempted to pack up to four templates per supercell (a 1 × 1 × 4 configuration of the crystallographic unit cell creating a onedimensional channel). Additionally, each template was initially positioned in one of five different orientations (as described above and in the Supporting Information) to confirm the results and to observe any effect of molecular shape. Figure 10 displays the interaction energies for each template in every orientation. Once again the increased volume of the ECHA template is clearly observed with the most favorable packing occurring at only two molecules per supercell, compared to three or four for DPBA and TPA. Moreover, the most stable interaction energy at optimal packing within the supercell (Table 7) is in the order DPBA > ECHA > TPA. Of course caution is necessary, as we are comparing kinetically controlled rates of reaction with the thermodynamic products of the energy minimization. However, Harris et al.43 have previously demonstrated a strong correlation between energetics of template “fit” with rates of crystallization for a series of zeolite structures, with the lower energy template fits resulting in increased rates of crystallization. Figure 11 plots the calculated interaction energy for each template system against both the rate of reaction and the activation energy. We see from this plot that the same correlation is observed in these AlPO systems, with the system with the lowest interaction energy having the greatest rate of reaction and correspondingly the lowest activation energy. Therefore, our computational and EDXRD results are consistent

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J. Phys. Chem. C, Vol. 111, No. 45, 2007 16959

Figure 12. Plot of interaction energies of TPA within the ATS framework (left) demonstrating that this is dependent on the ordering of the template within the supercell. When the template’s narrow cross section is pointing along the channel (YX and XY(45)ZX), ordering occurs resulting in packing of up to four templates per supercell; otherwise no ordering occurs (for example, YZ and YZ(45)ZX) (right).

TABLE 6: Size and Volume Measurements for Each Template Molecule Using the Maximum Dimensions Based on the Atom Nuclear Positions and Principal Axis Measurements nuclear size measurements

principal axis measurements

SDA

X (Å)

Y (Å)

Z (Å)

VdW vol (Å3)

RX (Å)

RY (Å)

RZ (Å)

mol vol (Å3)

TPA ECHA DPBA

9.226 9.509 6.061

6.436 7.410 6.920

2.817 5.488 4.834

175.94 239.71 193.57

5.096 11.177 6.498

4.573 3.953 3.227

2.545 3.351 3.099

134.265 183.426 145.749

TABLE 7: Lowest Interaction Energy (Einter) for Each Template within the ATS Supercell template TPA ECHA DPBA

no. templates per supercell

lowest Einter (kJ mol-1)

4 2 3

-352.14 -387.07 -452.51

and together demonstrate the effect of different templates on synthesizing the same framework. From these measurements we also note a second interesting feature. While the interaction energies for both ECHA and DPBA follow similar trends on increasing the packing density over all orientations, those for TPA do not. Specifically, packing up to four molecules per supercell can only occur when the TPA template is aligned in such a way that the short “Z” dimension is aligned along the length of the supercell channel (Figure 12). This orientation effect may also have some additional influence on the rate of formation of the TPAsynthesized materials as this reorientation facilitates the increased template concentration within the supercell, which results in the most favorable interaction energies. However, the effect needs to be studied in more detail as there is clearly little difference in the interaction energies between three and four templates in this case. We suggest that, in this case, it is perhaps necessary to achieve maximum template packing to allow the formation of the ATS framework, which manifests itself as a slow crystallization rate as the template molecules align themselves in the most favorable configuration. In contrast, in the case of ECHA, less critical packing of the template is required before crystallization can occur and moreover growth is less dependent on precise template packing.

Conclusions This work has focused on understanding how the type of organic template affects the underlying kinetics of formation of AlPO materials. EDXRD has demonstrated that the rate of formation is significantly altered depending on the template used to synthesize CoAPO-36, and crystallographic studies have also demonstrated that the ECHA-synthesized framework experiences an expansion in the crystallographic unit cell, which we proposed is due to the bulky nature of the template. Computational measurements have confirmed that ECHA does occupy a larger volume within the pores compared to the other templates. While the bulky nature of the template could explain the retardation in the crystallization of ECHA, it could not explain the slow formation of TPA. However, packing calculations have shown that the minimum interaction energy for TPA is less favorable than for both ECHA and DPBA, indicating that, as observed experimentally here, the TPA templated material should be the slowest to form. Finally, these experiments have revealed an orientation effect when packing the more planar TPA molecule within the AlPO framework. The importance of this effect needs to be investigated further as it may have consequences for the rate of formation of these materials and for our general understanding of their nucleation and growth. Acknowledgment. M.G.O. would like to thank Kevin Reeves for his assistance with the SEM imaging, Ian Watts for useful discussions, and the EPSRC for funding. M.S.-S. acknowledges the Spanish “Ministerio de Educacio´n y Ciencia” for a Ramon-y-Cajal contract. G.S., M.G.O. and M.S.-S. would like to thank D. Taylor, I. Harvey, and Daresbury laboratories for the use of stations 16.4 and 9.3.

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