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ARTICLES In Situ Comparison of Ionothermal Kinetics Under Microwave And Conventional Heating David S. Wragg,*,†,⊥ Peter J. Byrne,† Gae´tan Giriat,‡ Benjamin Le Ouay,† Ro´bert Gyepes,§ Andrew Harrison,‡,| A. Gavin Whittaker,‡ and Russell E. Morris† EaStCHEM School of Chemistry, UniVersity of St. Andrews, St Andrews, Fife, KY16 9ST, U.K., Centre for Science Under Extreme Conditions and School of Chemistry, The UniVersity of Edinburgh, The Joseph Black Building, King’s Buildings, Edinburgh EH9 3JJ, U.K., Department of Inorganic Chemistry, Charles UniVersity, HlaVoVa 2030, 128 40 Prague 2, Czech Republic, and Institut Laue LangeVin, 6 rue Jules Horowitz, BP 156, 38042 Grenoble Cedex 9, France ReceiVed: August 12, 2009; ReVised Manuscript ReceiVed: October 21, 2009
We have used in situ energy dispersive synchrotron X-ray diffraction to study the crystallization of aluminum phosphate frameworks under ionothermal conditions with conventional and microwave heating. The reaction is shown to follow slightly different routes depending on the type of heating used and a kinetic analysis shows that the rate constant is 10 times higher under microwave heating (1.4 compared to 0.14 min-1). The conventionally heated reaction is shown to proceed by a transformation of SIZ-3 to SIZ-4 via an intermediate, while the microwave-heated reaction forms SIZ-4 directly. The kinetic analysis is used to rationalize the differences in reaction rate and the findings are supported by SEM images of the crystal morphologies which result from the two heating methods. Introduction Framework synthesis using microwave heating can have several advantages over conventional methods, principally increased speed of synthesis.1 Ionothermal synthesis is the use of ionic liquids as both the solvent and template in the synthesis of crystalline inorganic and inorganic-organic hybrid solids.2 The ionic liquid (IL) solvents are good microwave absorbers,3,4 and their very low vapor pressure is of particular conveniences even heating to relatively high temperaturessproduces little in the way of autogenous pressure. This has some important consequences regarding safety and practicality of the experiment, and Yan and co-workers have demonstrated how microwave ionothermal synthesis can be used to great effect to produce high-quality zeolite coatings on metals for anticorrosion applications.5,6 The microwave method has also been extended to the ionothermal synthesis of metal organic frameworks.7 Another potential advantage of microwave heating is an increase in rate of crystallization when compared to conventional heating. Xu et al. reported that synthesis of aluminum phosphate molecular sieves by this method can be vastly accelerated by the use of microwave heating.8 In this paper we report a recent study of the synthesis of the ionothermally prepared microporous aluminum phosphate SIZ-49 (SIZ stands for St. Andrews Ionothermal Zeotype, a code used in our laboratory) in situ using energy dispersive X-ray diffraction (EDXRD) and compared the progress of the reaction and its kinetics under microwave * To whom correspondence should be addressed. E-mail: david.wragg@ smn.uio.no. † University of St. Andrews. ‡ The University of Edinburgh. § Charles University. | Institut Laue Langevin. ⊥ Current address: Centre for Materials Science and Nanotechnology, University of Oslo, Sem Sælands Vei, N-0315 Oslo, Norway.
and conventional heating. The low vapor pressure of ionic liquids is a major practical advantage over other synthetic methods for this type of in situ study in that the reaction does not need to be contained in pressure vessels, simplifying the experimental set up. The possibilities for this were recently illustrated by Zhang et al. in an in situ NMR study.10 EDXRD, in which a “white beam” of all X-ray wavelengths is used to produce diffraction patterns in which reflection positions are differentiated by energy, has been used in several studies of conventionally heated framework crystallization to study the kinetics and pathway of reactions.11-13 Its two great advantages are the size of sample which can be used (comparable to a normal synthetic batch) and the high time resolution possible (useful data can be collected on highly crystalline samples in under 10 s). These allow us to observe reactions in essentially their normal process conditions and to see changes which happen quite quickly. It has been shown that framework crystallization rate is highly dependent on the size of the reaction vessel meaning that the kinetics determined from angle dispersive studies in capillaries will not be equivalent to those in normal autoclaves.14,15 In some cases the high time resolution possible with EDXRD has allowed the discovery of intermediates in reactions which were subsequently isolated and fully characterized.16,17 At the time of writing there exist two in situ studies of zeolite crystallization under microwave heating.18,19 This team used a combined SAXS/WAXS (small/wide-angle X-ray scattering) setup at the National Synchrotron Light Source, Brookhaven with a custom waveguide capable of accepting 5 and 10 mm NMR tubes as reaction vessels to study the hydrothermal crystallization of several well-known zeolite phases and compare the microwave kinetics with those of conventionally heated reactions. Another interesting and relevant study was carried out by Jhung et al. Their method compared
10.1021/jp907785t CCC: $40.75 2009 American Chemical Society Published on Web 11/02/2009
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TABLE 1: Amounts of Reagents Used for Conventional and Microwave Heated Synthesis of SIZ-4a Al isopropoxide conventional MW a
H3PO4
HF
EMIM Br
g
mmol
g
mmol
g
mmol
g
mmol
H2O/mmol
0.4 0.1
2.0 0.5
0.68 0.17
6.1 1.5
0.06 0.015
1.4 0.3
10.37 5.7
54.3 29.8
11.7 2.9
The amount of water is estimated from the compositions of the H3PO4 and HF solutions.
the effect of conventional and microwave heating on the nucleation and growth stages of silicalite and VSB-5 formation by changing the heating method when the first XRD peaks appeared. Both nucleation and crystal growth were shown to be accelerated by microwave heating but nucleation much more so.20 Experimental Section EDXRD data were collected on station 16.4 of the Synchrotron Radiation Source, Daresbury, UK. The basic experimental setup consists of an un-monochromated beam of synchrotron light focused into the sample with diffracted beam collected by a three element detector21 which is positioned in order to maximize the intensity of the main Bragg peaks in the diffraction pattern (in this case the 2θ position of the bottom detector was 1.435°). In the conventionally heated experiments the heat source was a custom-made block heater controlled by a standard Eurotherm unit. This general setup is fully described elsewhere.22 Diffraction patterns were collected with an accumulation time of 2 min. The detector angles were calibrated against a sample of SIZ-4. The microwave heater comprised a commercial microwave source (ASTEX AX2110, output power 20-1000 W) to generate microwave radiation at 2.45 GHz. During data collection, the temperature was continuously monitored using a fiber-optic probe (Neoptix T1, resolution of 0.1 K) in intimate contact with the sample mixture. A computer was used to control the temperature by varying the microwave power sent to the sample based on the output of the fiber-optic probe. The microwave radiation was driven to the sample through a WR284 (WG10) waveguide operating in the fundamental TE10 mode. This waveguide was also the microwave cavity with a sliding short termination driven over a 12 cm range by a remotely controlled linear actuator. This arrangement allows the system to be finely tuned by adjusting the cavity length, hence permitting control of heating efficiency in the sample. The sample holder was held vertically in the center of the waveguide. On either side of the sample two windows are cut from the cavity walls; a small hole located on the incident X-ray beam side and on the opposite wall, a larger aperture covered with thin aluminum foil (∼10 µm), designed for the three element detector geometry, permits passage of the scattered X-rays. The above setup is incapable of delivering zero microwave power while under computer control (the minimum output is 30 W). In order to prevent overheating of the IL we used a flow of water as a dummy load for the microwaves. A hole was bored through the waveguide to allow a glass tube containing flowing water to be passed through. In early experimental runs with the microwave setup we found that the three-element detector was extremely sensitive to low levels of microwave leakage from the cavity. To minimize the interaction extra microwave shielding (aluminum foil) was added around the cavity and detector assembly (see Supporting
Information). The X-ray beam of station 16.4 is powerful enough to penetrate the aluminum shielding with minimal loss of intensity. Ionothermal synthesis seems ideal for in situ study as the reactions can be carried out in normal glassware without sealing. One problem, however, is that the concentration of reagents which can be used is very low compared to hydrothermal synthesis since it is easy to add so much water with the two acids that its molar quantity is greater than that of the IL. In order to observe diffraction from the crystallizing product in our reaction setup it was necessary to increase the concentration of reactants above that normally used in ionothermal synthesis. However, we were always able to remain within the ionothermal regime (molar ratio of less than 1:1 water/IL). Another consequence of the necessarily low reactant concentration was that the reaction could not be stirred if diffraction from the product was to be observed. This leads to a slight reduction in the reaction rate compared to our normal synthetic laboratory microwave setup (a Biotage Initiator 8) and, more importantly, some variation in peak intensity due to convection currents lifting crystalline material in and out of the beam (similar results were observed by Geselbracht and co-workers23 when studying unstirred flux reactions using station 16.4). Microwave reactions were carried out in 8 mm diameter glass test tubes with a fiber optic temperature probe lowered into the reaction mixture and the top sealed with a Teflon plug and tape. Conventionally heated reactions were contained in sealed 2-5 mL size Biotage glass pressure-containing reaction vials. Aluminum isopropoxide (Aldrich) was mixed with phosphoric acid (85% weight solution in water, Aldrich) in the chosen reaction vessel and hydrofluoric acid (48% weight solution in water, Fischer) was dropped onto the mixture with a plastic pipet. 1-Ethyl-3-methylimidazolium bromide (EMIM Br) synthesized as described by Cooper et al.9 was added, and the container was sealed and transferred either to the microwave cavity or the heater carousel and heated at 180 °C. A ramp rate of 60 °C/min was used for the microwave heating and for the conventional experiments the heater temperature was increased to 180 °C before the reaction tube was inserted. The amounts of reagents used for microwave and conventional reactions are given in Table 1 below. In order to observe diffraction from the product phases it was necessary to aim the X-ray beam just above the solid aluminum isopropoxide in the initial solid mixture before heating (note that while liquid under ionothermal conditions EMIM Br is a solid at room temperature, mp 83 °C). Observations made during test experiments with the microwave cavity used in this work show that the aluminum isopropoxide is gradually dissolved into the ionic liquid during the reaction while the product remains suspended in it after crystallization. We suggest that the best signal is observed just above the dissolving aluminum source as this is where the crystallizing product is most densely suspended. The final mixtures after several hours of heating contain very little sediment, the product being suspended in the ionic liquid.
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Figure 1. Three-dimensional plots of the XRD data from the bottom detector for conventionally heated SIZ-4 synthesis showing the initial SIZ-3 (0 2 0) peak at 53.4 KeV turning to SIZ-4 (1 0 0) at 54.6 KeV. The large peak at 51.2 KeV is thought to be due to a transient intermediate phase. The full detector range is shown on the left to demonstrate the unique final phase.
Extraction of the peak intensities for kinetics plots was carried out with the program Fityk24 using a script creator interface developed by O’Brien.25 Scanning electron microscope images were collected on a Jeol JSM5600. Results and Discussion Conventional Synthesis of SIZ-4. The in situ EDXRD patterns for synthesis of SIZ-4 with conventional heating show strong peaks in the high d-spacing region covered by the bottom element of the three-element detector (figure 1). Some reflections are also visible later in the reaction on the middle detector element which can be unambiguously assigned to SIZ-4. The main features observed on the bottom detector begin to appear after about 80 min with a peak at 53.4 KeV (d-spacing ) 9.26 Å) which can be attributed to the (0 2 0) reflection of SIZ-3. After ∼3 h a new peak at 54.6 KeV (dspacing ) 9.06 Å) appears and begins growing in intensity. This peak corresponds to the (1 0 0) peak of SIZ-4. By the end of the experiment only the SIZ-4 (1 0 0) peak is present. As this peak begins to grow in intensity we also see the first appearance of an extremely intense peak at an energy of 51.2 KeV (d ) 9.66 Å). This peak is present in three or four consecutive patterns (6-8 min of reaction time) then fades away again, reappearing from time to time in a similar short-lived manner. The d-spacing of this peak corresponds to the most intense peak in the powder pattern of AlPO-CJ826 a ladderlike chain phase which is built from four ring units similar to those seen in SIZ-3 and 4. The conversion of SIZ-3 to SIZ-4 has not previously been observed, although transformation from AlPO-5 to SIZ-3 and subsequently to AlPO-25 has been observed by Wang and coworkers in ionothermal synthesis with 1-butyl-3-methylimidazolium bromide and amine templates.27 The existence of this transformation explains some of the difficulties encountered in our attempts to influence production of either SIZ-3 or SIZ-4 from this system by manipulating the water content.22 Further ex situ investigation of the reaction by quench cooling of reactions at various stages of heating has led to the isolation of mixtures of SIZ-3 and 4 at intermediate reaction times showing that the change from one phase to the other is quite slow. We have as yet been unable to isolate the intermediate phase and so cannot unequivocally identify it; we do, however, point out that the template found in the structure of AlPO-CJ8 also directs
the formation of the layered material AlPO-CJ1228 when used in solvothermal synthesis without hydrofluoric acid. We have found that AlPO-CJ12 can be prepared in a solvothermal reaction with 1-methylimidazole (the amine used to prepare EMIM Br) as solvent and template. In the same study we found evidence that under ionothermal and fluoride containing conditions 1-methylimidazole is produced from EMIM Br and helps to template SIZ-4.29 We therefore suggest there is reasonable evidence that AlPO-CJ8 is the intermediate phase (there are no other materials with this d-spacing in the AlPO database30). We also note that layered intermediates have been observed in the hydrothermal synthesis of the chabazitic material SAPO-34.31 A further operational consideration for this type of study is that we noticed small but significant X-ray burns in the IL solvents after the reaction. The burns were visible as dark brown streaks through the solidified IL on cooling of the reaction mixtures suggesting both that some of the solvent may have decomposed in the X-ray beam and that local heating may occur because of the beam. Microwave Synthesis of SIZ-4. EDXRD shows that the microwave synthesis of SIZ-4 follows a significantly different route to that found with conventional heating. Even with a relatively slow heating ramp (compared to our lab microwave synthesis system, which heats the same synthesis mixture to 180 °C in around 45 s) the induction time for crystallization is reduced to only 30 min. The main peaks are observed in the range of the bottom detector (Figure 2). The first feature to appear is the SIZ-4 (1 0 0) peak which remains the strongest feature until its intensity is constant and the reaction is complete. There is no sign of either of the other phases observed in the conventionally heated reaction. This result is similar to those of Panazarella et al. who have shown that the selectivity of the synthesis of a mixture of sodium template aluminosilicate zeolites A, X and sodalite is pushed toward the NaA and NaX under microwave heating. The change seems to be related to a more rapid onset of crystallization,18 which is in agreement with the findings of Jhung and coworkers.20 In the microwave data the weaker (1 1 0) peak of SIZ-4 is also observed in the EDXRD pattern at 80.3 KeV (d-spacing ) 6.34 Å) suggesting superior crystallinity to the conventionally heated reaction. This could also be due to the increased number and wider distribution of nucleation sites throughout the reaction
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ln[-ln(1 - R)] ) n ln(t) + n ln k
(2)
Using a range of R from 5% to 90%, as in previous studies of this type15,37 to look at a section of the crystallization where the growth rate is essentially linear, Sharp-Hancock plots were made (Figure 4) and the values of the rate constant (n, slope) and reaction order (k, x-axis intercept) for the reactions were obtained (Table 2). There is a suggestion in the microwave data that the reaction could progress in two stages with the first (up to ln(t) ) -0.4) having a slightly greater slope (higher reaction rate). As this is a vague feature in a small number of data points and no evidence of rate changes during similar crystallizations has previously been observed by other researchers we have chosen to discount it. Hulbert38 in a review of models of solid state phase growth suggests that n is indicative of the growth mechanism of the Figure 2. Three-dimensional plot of the EDXRD data for the microwave synthesis of SIZ-4 showing the direct formation of the final product with excellent crystallinity. Both the (1 0 0) and (1 1 0) peaks are observed, in contrast to the conventionally heated reaction (see Figure 1).
mixture under microwave heating which is discussed further below. The intensity of the (1 0 0) peak is rather variable with several extremely high peaks occurring during the experiment; this can be attributed to convection currents moving crystalline material in and out of the beam as described in the experimental section above and in ref 23. We believe that the high viscosity of ionic liquids accentuates this effect. There is also a possibility that this could be caused by preferred orientation of the crystallites. Reaction Kinetics. Due to the SIZ-3 (0 2 0) and SIZ-4 (1 0 0) peaks being very close together and difficult to separate when one or the other is at low intensity, the kinetics of SIZ-4 formation with conventional heating were followed using the (1 -2 1) reflection which can clearly be identified in the middle detector patterns and does not overlap with any SIZ-3 reflections. In doing this we make the assumption that the intermediate phases observed have no effect on the rate of formation of SIZ4. Since it is possible to measure the development of a peak specific to SIZ-4 we believe that this offers a sensible comparison with the direct formation rate of the microwave synthesis. Since there was no other phase present in the microwave SIZ-4 synthesis the large (1 0 0) peak was used to determine the progress of the reaction. As observed above the variation in intensity of the peaks in the microwave pattern is probably due to convection currents moving the product crystals in and out of the beam or preferred orientation. The logarithms of the extracted peak areas were plotted against time (Figure 3) and a final value of 100% product was determined by averaging the peak areas for points along the final horizontal section of the reaction curves. This end point is reached after 80 min in the microwave reaction and ∼11 h with conventional heating. By expressing the peak area values as fractions of the “100% product” figures we obtained conversion factors (R) for the two reactions. The Avrami equation32-34 which has been shown to offer a good model for zeolite crystallization35 can be expressed in terms of R (eq 1) and can be converted into a linear form by taking natural logarithms of both sides. This gives the Sharp-Hancock equation (eq 2).36
-ln(1 - R) ) (kt)n
(1)
Figure 3. Plots of the logarithm of the areas of the (1 -2 1) (conventional heating) and (1 0 0) (microwave heating) peaks against time illustrate the progress of the synthesis of SIZ-4 under conventional and microwave heating. Variations in the intensity of the microwave (1 0 0) peak are due to convection currents moving crystalline material in and out of the beam. Note that due to the logarithmic scale the points where the peak area is zero prior to the start of crystallization do not appear in these plots. Time ) 0 represents the start of heating in both reactions.
Figure 4. Sharp-Hancock plots for conventional and microwave synthesis of SIZ-4. The rate constant is derived from the slope and the reaction order from the x-axis intercept of the linear regression lines.
TABLE 2: Values of Reaction Order (n, Derived from the x-Axis Intercept) and Rate Constant (k, Derived from the Slope) for Conventionally and Microwave Heated Synthesis of SIZ-4 from Linear Fits of the Sharp-Hancock Plots in Figure 4 microwave conventional
slope
intercept
K (min-1)
n
5.6(4) 1.63(9)
1.8(2) –3.1(2)
1.4 0.14
5.6 1.63
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Figure 5. SEM images at 270× magnification of the SIZ-4 produced by microwave (left) and conventionally heated (right) Ionothermal syntheses. The small spherical crystals produced by the rapidly nucleating microwave reaction contrast with the larger crystallites prepared at a more gentle rate with conventional heating.
crystallites. The n value of 1.63 for the conventionally heated reaction corresponds to the suggested values for phase boundary controlled one-dimensional growth with a decreasing nucleation rate or two-dimensional growth diffusion controlled growth with a decreasing nucleation rate. For the microwave heated reaction n is 5.6; this would suggest extremely fast phase boundary controlled growth in three dimensions (the speed of the microwave heated reaction compared to the conventional version is illustrated by the k values of 0.14 min-1 for conventional and 1.4 min-1 for microwave, 10 times greater). SEM images (Figure 5) of the products of the microwave and conventionally heated reactions help us to interpret this further. Viewed at the same level of magnification the crystallites of conventionally heated SIZ-4 resemble flat plates while the microwave product looks more spherical. We conclude therefore that the mechanism of conventionally heated SIZ-4 synthesis is two-dimensional diffusion-controlled growth with a decreasing nucleation rate (i.e., as the crystallites grow the number of nucleation sites declines, fitting the SEM observation of relatively large, plate-like crystals) while under microwave heating an extremely fast phase boundary controlled mechanism involving a huge and increasing number of nucleation sites for three-dimensional growth occurs. The large number of nucleation sites may be due to the rapid nature of microwave heating of the IL solvent allowing new nucleation sites to appear as soon as the boundary growth rate of a crystallite cannot keep up with the reaction. Another possibility is that “pockets” of water in the ionic liquid are subjected to extremely rapid heating. Our observation of increased nucleation rates under microwave heating is consistent with the findings of Jhung et al.20 Conclusions This paper reports the first in situ study of ionothermal kinetics and has revealed significant differences between the mechanisms with conventional and microwave heating. The unexpected discovery that SIZ-3 can be converted to SIZ-4 during the synthesis of the latter helps to explain the prevalence of SIZ-4 in our study of the influence of water on the synthesis of the two phases.39 The discovery of the highly crystalline and short-lived intermediate phase which appears as SIZ-3 is replaced by SIZ-4 as the dominant phase shows that the processes which take place in ionothermal synthesis can be extremely complex although SIZ-4 can be prepared in a single step with microwave heating. This may be due to the speed and consistency of microwave heating, which lead to an increased number of nucleation sites and many small crystals. Continued in situ study of ionothermal reactions will undoubtedly reveal more interesting features of this type of synthesis.
Note Added after ASAP Publication. This article was published ASAP on November 2, 2009. The name of the second author has been corrected. The correct version was published on November 5, 2009. Acknowledgment. We thank the Engineering and Physical Sciences Research Council for funding and the Science and Technology Facilities Council for access to the SRS. We also thank Dr Matt O’Brien (University of Utrecht) for useful discussions and providing a copy of his Fityk script creator. We thank Prof Poul Norby (Risø Institute) for discussions. Supporting Information Available: Pictures of MW setup on station 16.4. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Tompsett, G. A.; Conner, W. C.; Yngvesson, K. S. Chem Phys Chem 2006, 7, 296–319. (2) Parnham, E. R.; Morris, R. E. Acc. Chem. Res. 2007, 40, 1005. (3) Hoffmann, J.; Nu¨chter, M. N.; Ondruschka, B.; Wasserscheid, P. Green Chem. 2003, 5, 296. (4) Leadbeater, N. E.; Torenius, H. M. J. Org. Chem. 2002, 67, 3145. (5) Cai, R.; Sun, M. W.; Chen, Z. W.; Munoz, R.; O’Neill, C.; Beving, D. E.; Yan, Y. S. Angew. Chem., Int. Ed. 2008, 47, 525–528. (6) Morris, R. E. Angew. Chem., Int. Ed. 2008, 47, 442–444. (7) Lin, Z. J.; Wragg, D. S.; Morris, R. E. Chem. Commun. 2006, 2021– 2023. (8) Xu, Y; Tian, Z.; Wang, S.; Hu, Y.; Wang, L.; Wang, B.; Ma, Y.; Hou, L.; Yu, J.; Lin, L. Angew. Chem. Int. Ed. 2006, 118, 4069–4074. (9) Cooper, E. R.; Andrews, C. D.; Wheatley, P. S.; Webb, P. B.; Wormald, P.; Morris, R. E. Nature 2004, 430, 1012. (10) Xu, R.; Zhang, W.; Guan, J.; Xu, Y.; Wang, L; Ma, H.; Tian, Z.; Han, X.; Lin, L.; Bao, X. Chem.sEur. J. 2009, 15, 5348–5354. (11) Francis, R. J.; O’Hare, D J. Chem. Soc., Dalton Trans. 1998, 3133– 3148. (12) Walton, R. I.; O’Hare, D. Chem. Commun. 2000, 2283–2291. (13) O’Hare, D.; Evans, J. S.; Francis, R. J.; Shiv Halasyamani, P.; Norby, P.; Hanson, J. Microporous Mesoporous Mater. 1998, 21, 253– 262. (14) Norby, P. Curr. Opin. Colloid Interface Sci. 2006, 11, 118–125. (15) Grizzetti, R.; Artioli, G Microporous Mesoporous Mater. 2002, 54, 105–112. (16) Walton, R.; Loiseau, T.; O’Hare, D.; Ferey, G. Chem. Mater. 1999, 11, 3201–3209. (17) Millange, F.; Walton, R.; Guillou, N.; Loiseau, T.; O’Hare, D.; Ferey, G. Chem. Mater. 2002, 14, 4448–4459. (18) Panzarella, B.; Tompsett, G.; Conner, W. C.; Jones, K. Chem. Phys. Chem. 2007, 8, 357–369. (19) Tompsett, G. A.; Panzarella, B. A.; Conner, W. C.; Bennett, S.; Jones, K. W. Nucl. Instrum. Methods Phys. Res. Sect. B 2007, 261, 863– 866. (20) Jhung, S.; Jin, T.; Hwang, Y.; Chang, J. Chem.sEur. J. 2007, 13, 4410–4417. (21) Muncaster, G.; Davies, A. T.; Sankar, G.; Catlow, C. R. A.; Thomas, J. M.; Colston, S. L.; Barnes, P.; Walton, R. I.; O’Hare, D. Phys. Chem. Chem. Phys. 2000, 2, 3523–3527.
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