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The Effect of Heating Rate and Gas Atmosphere on Template Decomposition in Silicalite-1 Katherine H. Gilbert, Robert M. Baldwin,* and J. Douglas Way Chemical Engineering Department, Colorado School of Mines, Golden, Colorado 80401
Template evolution from TPABr/silicalite powder was studied in a thermogravimetric analyzer as a function of heating rate and gas atmosphere to determine the maximum rate of weight loss and the temperature at which the rate of weight loss was a maximum. Through the application of TPD theory, the activation energies of the thermal decomposition reactions in various gaseous atmospheres (helium, oxygen, air, ozone/air mixture) were also calculated. The results of these investigations showed that an increased calcination heating rate had a direct correlation with an increased rate of weight loss. The template removal reaction in an inert atmosphere (helium) was endothermic; however, the presence of oxidizing components in the gas atmosphere gave rise to significant exothermic reactions. We found that the least stressful conditions for template removal occurred with a heating rate of 0.5 K/min in an atmosphere containing a mixture of air and ozone (1%). For these conditions, the maximum rate of weight loss was 0.061%/min, and the temperature at maximum weight loss was 487 K; the template was fully removed at a final temperature of ca 723 K. I. Introduction Reproducible synthesis of defect-free zeolite or other membranes involving crystalline materials is difficult because of the presence of nonzeolite pores and alternate diffusion pathways that are formed during either the synthesis or calcination process. These defects are most often presumed to be caused by heat- and/or pressureinduced stresses, which are created within the zeolite film as the organic template used as the structuredirecting agent thermally decomposes.1 The large variation in selectivities reported for the separation of gaseous mixtures using ZSM-5 and silicalite membranes has been attributed at least in part to the size and number of defects in these films.2 Typically, the synthesis of MFI-type zeolites involves use of an organic template that acts as a structure-directing agent during crystallization. To open the pores of the zeolite, the template is removed by thermal degradation of the organic molecule. During this process, called calcination, the products of template decomposition must diffuse through the zeolite channels. However, cracks within the crystals themselves or within the film are often created as the template is removed. These cracks are defects in the zeolite membrane that create pathways larger than zeolitic pores and are detrimental to the performance of the membrane. Structural stresses on the crystals are created during calcination by thermal effects and by pressure-induced stresses arising from the large volumetric changes associated with decomposition (solid f gas) of the template. As the temperature of silicalite is increased, the crystal structure shrinks, and upon cooling, the structure expands.3,4 The cooling of zeolite membranes after template removal was also found to influence crack formation through interactions of the support/zeolite layer.5 By decreasing the temperature of the template removal reaction, these stresses are minimized. Crystal cracking has been attributed to the strain produced by * Corresponding author. E-mail:
[email protected].
trapped gaseous products unable to diffuse out of the zeolite pores.6 A low rate of weight loss during calcination would hence reflect a more gradual process and, therefore, a low amount of stress on the crystal structure. A common template used in the synthesis of ZSM-5 and silicalite is tetrapropylammonium ion (TPA).7-9 TPA decomposition has been analyzed by thermoanalytical techniques (TG, DTA, DSC),10-13 gas chromatography,14 mass spectrometry, and 12,14 NMR11,13 and IR spectroscopies.13,15,16 Temperature profiles used for thermal calcination reported in the literature for template removal from silicalite and ZSM-5 vary. Generally, zeolites are heated to temperatures between 673 and 873 K at a heating rate ranging from 0.1 to 15 K/min in an inert or air atmosphere. Peterson et al. proposed a discontinuous heating profile, which provides a continuous and slow weight loss over a long period of time.17 The profile utilizes a fast heating rate at low temperatures, followed by a heating rate of 0.1 K/min from 598 to 613 K. A “safe” calcination profile developed by Geus and van Bekkum recommended a heating rate of 1 K/min to a maximum temperature of 673 K in air to avoid cracking of the crystals.6 Lower heating rates were found to result in lower rates of weight loss,10 which were hypothesized to create less strain on the zeolite crystal. In an attempt to better understand the factors controlling the calcination process for silicalite membranes, we used silicalite powder to investigate the effect of heating rate and calcination gas atmosphere on the template decomposition reaction. To quantify these effects, we considered two parameters: (1) the rate of weight loss and (2) the temperature at maximum weight loss rate (Tmax rate). These parameters were used to define and compare the conditions under which the template decomposed. The goal of these experiments was to find the conditions that provide the least stressful reaction conditions for template removal from silicalite powder.
10.1021/ie010069v CCC: $20.00 © 2001 American Chemical Society Published on Web 10/09/2001
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Figure 1. TG/DTG/DTG data of template-only decomposition in helium with a 5 K/min heating rate.
II. Experimental Section The synthesis solution for silicalite powder was a modified ZSM-5 recipe taken from Jacobs et al.18 The synthesis solution, 1.71 Na2O:24 SiO2:1.14 TPABr:500 H2O, was heated at 453 K in a Teflon-lined 75-mL Parr autoclave for 18 h. After synthesis, the crystals were rinsed with deionized water, filtered, dried at atmospheric conditions for 12 h, and then dried at 373 K for 12 h. The crystal size was around 10 µm as determined by SEM. Analysis by XRD confirmed that silicalite was formed. A more detailed description of the synthesis procedures can be found elsewhere.16 A Seiko TG/DTA 220 thermogravimetric analyzer (TGA) coupled to a Fisons-VG quadrupole residual gas analysis (RGA) mass spectrometer was used to investigate the effect of heating rate and gas atmosphere on the template decomposition temperature and rate and to speciate the decomposition products. Template decomposition experiments in the TGA were performed in four different gas atmospheres, namely, helium, air, oxygen, and a mixture of ozone and air, with four different continuous heating rates of 0.5, 5, 10 and 15 K/min. Each of the continuous heating profiles began at 303 K and heated at the designated heating rate to 873 K. The temperature was held constant at 873 K for 30 min and then decreased at a rate of 10 or 15 K/min to room temperature. Silicalite sample weights were between 15 and 25 mg, and quartz chips were used as reference samples for the DTA measurements. For all experiments, the flow rate of gas was 150 mL/min over the sample. For experiments using ozone, oxygen was fed to an ozone generator in a Thermo Electron Chemiluminescent Series 44 NO-NOx Analyzer. Makeup air was added to the ozone/oxygen gas mixture to achieve the desired total gas flow rate to the TGA. The amount of ozone in the mixed gas feed was approximately 1%. III. Results and Discussion As described above, the temperature at which the rate of weight loss is a maximum and the maximum rate of weight loss are two parameters that can be used to
describe the “stress” on the zeolite film during the template decomposition reaction. As the template decomposes, the decomposition products must diffuse out of the zeolite channels. However, as the temperature increases, the pore diameter and volume decrease, thereby increasing the resistance to diffusion for template decomposition products. Therefore, by finding the lowest maximum rate of weight loss and the lowest temperature at the maximum weight loss rate, the conditions corresponding to the least stress for calcination can be determined. The assumption that the temperature at maximum weight loss is reproducible was essential to our analysis and was confirmed by performing several experiments under the same conditions. Five experiments were done in helium at a 10 K/min heating rate, and four experiments in oxygen at 5 K/min heating rate. The standard deviation for the temperature at maximum weight loss rate was calculated to be 0.48 K for helium environments and 3.31 K in an oxygen atmosphere. All experimental runs were done in duplicate, and the differences were well within the standard deviation found from the reproducibility runs. Sample size did not effect results, as was also found by Lee and Hayhurst.10 The first series of experiments investigated the decomposition of the template molecule alone, not occluded in silicalite. Figure 1 shows TG/DTG/DTA data from the decomposition of [TPA]+[Br]- in helium. From the endothermic DTA peak, it was evident that TPABr in helium decomposed by a pyrolysis mechanism. Results shown in Table 1 for the template alone indicate that, for each atmosphere, an increase in the heating rate corresponded to a higher temperature of reaction and a faster rate of weight loss. Differential thermal analysis on TPABr indicated that decomposition of the template molecule in the absence of zeolite was an endothermic process regardless of the composition of the gas atmosphere (inert or oxidizing). A second series of experiments investigated the template decomposition reaction in silicalite crystals. The results in Table 2 show effects for heating rate and atmosphere that are similar to those found for the decomposition of the template molecule alone. For each
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Table 1. Thermal Decomposition Data at Various Heating Rates and Atmospheres for TPABr Only heating rate (K/min)
temperature at maximum weight loss (K)
maximum rate of weight loss (%/min)
helium
0.5 5 10 15
503.0 510.2 538.0 543.3
15.76 27.25 43.1 56.45
air
0.5 5 10 15
488.5 507.8 539.0 557.0
9.71 23.2 39.7 70.25
oxygen
0.5 5 10 15
498.2 522.4 531.0 537.1
3.51 33.7 48.6 60.4
atmosphere
Table 2. TG Data at Various Heating Rates and Atmospheres for Silicalite Template Decomposition heating rate (K/min)
temperature at maximum weight loss (K)
maximum rate of weight loss (%/min)
helium
0.5 5 10 15
608.5 666.2 677.7 681.4
0.173 1.78 3.88 5.05
air
0.5 5 10 15
637.6 666.9 678.5 684.9
0.290 4.15 7.33 10.33
oxygen
0.5 5 10 15
648.4 678.0 702.0 720.8
0.412 10.14 25.47 39.61
ozone and air
0.5 5 10 15
487.4 673.4 687.8 710.45
0.061 5.79 14.10 31.83
atmosphere
atmosphere, the heating rate was found to correlate directly with the rate of weight loss and the temperature at maximum weight loss rate. As expected, the lowest heating rate gave the lowest rate of weight loss for each atmosphere. The helium atmosphere provided the lowest temperature of reaction (Tmax rate) for all heating
rates except 0.5 K/min, for which the combined ozone/ air atmosphere gave the lowest value for Tmax rate. At the lowest heating rate (0.5 K/min), the effect of gas atmosphere on the rate of weight loss was essentially negligible, ranging from approximately 0.1%/min in the ozone/air mixture to 0.4%/min in oxygen. However, at the high heating rate, the effect of the gas atmosphere on the rate of weight loss was pronounced, varying from 5.05%/min in helium to 39.6%/min in oxygen. As can be seen, the effect of gas atmosphere becomes a more important variable in the calcination process at higher heating rates. This is probably due to the differences in the decomposition mechanisms involved (pyrolysis vs pyrolysis/combustion). Figure 2 shows TG data for template decomposition in silicalite at a 5 K/min heating rate in a helium atmosphere. The weight loss curve indicates an initial 1.0% weight loss due to dehydration or desorption of other contaminants at low temperatures in the range of 373-473 K. This is followed by a rapid weight loss corresponding to the decomposition of the TPA template.19 Above 873 K, the remaining silicalite structure is stable, and no further weight loss is observed until 1273 K.10 Although not apparent, the decomposition process takes place in two global steps (viz., the slight change in the slope of TGA weight loss curve). For purposes of this paper, all conclusions presented in Table 2 are based on the first weight loss event, as this was at which the rate of weight loss was the greatest and hence reflects the conditions where the zeolite crystals were under the greatest stress. From Figure 2, it is evident that the decomposition of TPA occluded in silicalite in a helium atmosphere is endothermic. All TPA/silicalite decomposition reactions in helium were endothermic; however, in air, oxygen, and ozone/air atmospheres, the reactions were found to be net exothermic. The absence of exothermic reactions for template-alone decomposition reactions in the presence of oxygen indicates that silicalite might be catalyzing oxidation reactions for the template or for fragments of the template produced via initial pyrolysis events. A reaction consisting of a physical mixture of calcined silicalite and template was performed, but these data were essentially identical to those obtained
Figure 2. Template decomposition in silicalite in helium at a 5 K/min heating rate.
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Figure 3. Template decomposition in silicalite in ozone/air at a 0.5 K/min heating rate.
with TPABr alone (Table 1). This observation does not appear to support the hypothesis of catalysis of template oxidation; however, it is likely that the mechanism for decomposition of the template when occluded in silicalite is significantly different. In addition, the delayed release of template decomposition products for TPABr occluded in silicalite exposes these products to significantly higher (ca 150 K) temperatures, as can be seen by comparing the data in Tables 1 and 2, possibly engendering combustion reactions because of the higher temperatures. The lowest rate of weight loss for all reactions in Table 2 occurred in ozone/air at a heating rate of 0.5 K/min. There were two distinct exothermic peaks and one plateau in the weight loss curve, as seen in Figure 3. The first weight loss cannot be attributed solely to dehydration as the change in weight indicated a ∼7% weight loss. In an inert atmosphere, it could be speculated that the first weight loss event before the plateau was a result of the rate-limiting step of the pyrolytic decomposition reaction. Through IR spectroscopy15 and mass spectrometry12,16 studies, the second step, the decomposition of the tripropylamine, was found to be the rate-limiting step for TPA decomposition.15 Therefore, the observed 7% weight loss could be attributed to the first step of the pyrolytic decomposition mechanism and the diffusion of tripropylamine,16 with the remaining weight loss attributed to further decomposition. Weight loss data from Soulard et al. also showed a very slight plateau in the TG curve that corresponded to a 7 wt % change in argon for a 6 K/min heating rate.20 The appearance of the plateau was attributed to the ratedetermining step of the decomposition reaction. As seen in Figure 3, the presence of an ozone/air mixture produced an exothermic reaction. This was also the case for the template removal reactions in air and oxygen when the template was occluded in the silicalite crystals. On-line mass spectrometry of silicalite calcination in air, oxygen, and ozone/air indicated the production of carbon dioxide and water; these combustion products were not observed for silicalite calcination in an inert atmosphere. The energy of the exothermic events also increased the bulk temperature of the silicalite (as measured by the TGA pan thermocouple), suggesting the possibility of significant localized heat-
ing. This effect was very evident when the sample temperatures at reaction initiation and maximum weight loss and the final temperature of reaction were compared. The initial and final temperatures of reaction were determined by recording the temperature when the rate of weight loss reached some arbitrary threshold value (i.e., reaction of initiation began when weight loss was greater than 0.2%/min). For template removal in an oxidizing atmosphere, it was found that the bulk silicalite temperature at maximum weight loss was fully 25 K higher (for the 15 K/min heating rate) than the final temperature of reaction. This sudden increase in temperature not only creates additional thermal stress on the crystal structure, but also constricts the effective pore size. The role that oxygen plays in the decomposition mechanism is unclear. These experiments suggest that template decomposition in the presence of zeolite performed in air, oxygen, and ozone/air environments occurs by a combination of pyrolysis and oxidation. It is possible that the template is oxidized while it is occluded in the silicalite or that oxidation of the evolving template decomposition products takes place, possibly catalyzed by the zeolite. Figure 4 shows a comparison of the weight loss curves of TPA decomposition for the 0.5 K/min heating rate in air, helium, and ozone/air atmospheres. A plateau in the weight loss curve for low heating rates was observed for helium, air, and ozone/air; however, the plateau was much more distinct for the ozone/air atmosphere. In general, this plateau was better defined at lower heating rates. As can be seen from Figure 4, template decomposition in ozone/air atmosphere started at temperatures that were approximately 100 K lower than in air or helium, and the weight loss also occurred more gradually than in all other atmospheres. Temperature-programmed desorption (TPD) theory was used to calculate the activation energy for silicalite template decomposition from the rate of weight loss and temperature at maximum weight loss data following the development of Lee and Hayhurst.10 TPD can be used to compare the activation energies in different atmospheres to determine whether the same mechanism of decomposition takes place in the each atmosphere. For purposes of estimating the activation energy, the bulk
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Figure 4. Weight loss for template decomposition in silicalite in air, helium, and ozone/air at a 0.5 K/min heating rate.
sample temperature associated with the rate of maximum weight loss for the first weight loss event was used in all calculations. The calculated activation energies for template decomposition were 49.58 kcal/mol in helium, 59.83 kcal/mol in air, 36.59 kcal/mol in oxygen, and 7.13 kcal/mol in ozone/air. Lee and Hayhurst reported an average activation energy of 51.4 kcal/mol but did not specify whether the reactions were performed in nitrogen or air. The activation energy for helium is higher than that for oxygen, presumably because oxidation reactions typically are not as temperature-sensitive as pyrolysis reactions. The differences found in activation energies for silicalite template decomposition in inert and oxidizing environments indicate that the template decomposes by different mechanisms in these atmospheres. This is consistent with the differential thermal analysis data, which also indicate two different mechanisms, endothermic or exothermic, depending on the atmosphere. IV. Conclusions The mildest conditions for thermal calcination of TPABr/silicalite were found to be a heating rate of 0.5 K/min in an atmosphere of air (99%) and ozone (1%). These conditions gave rise to both the lowest maximum rate of weight loss and the lowest temperature at the point of maximum rate of weight loss. The decomposition reaction in the air/ozone mixture was also found to initiate at a temperature approximately 100 K lower than in other gaseous atmospheres. In general, lower heating rates correlated with lower rates of weight loss. Activation energies calculated by TPD theory indicated that the calcination atmosphere does have an effect on the template decomposition reaction mechanism. However, it is unclear exactly what role oxidizing species play in the calcination process. Significant exothermic events were noted when oxidizing gas atmospheres were used for template removal. This observation suggests that inert gases should be used during calcination to avoid stress cracking of the film and/or zeolite crystals due to localized heating.
Acknowledgment Support for this work from the Center for Commercial Applications of Combustion in Space at the Colorado School of Mines under NASA Cooperative Agreement NCCW-0096 is gratefully acknowledged. J.D.W. is supported by the U.S. Department of Energy, Office of Science, through Grant DE-FG03-93ER14363. Literature Cited (1) Matsukata, M.; Kikuchi, E. Zeolitic Membranes: Synthesis, Properties and Prospects. Bull. Chem. Soc. Jpn. 1997, 71, 23412356. (2) Yan, Y.; Davis, M. E.; Gavalas, G. R. Preparation of highly selective zeolite ZSM-5 membranes by a postsynthetic coking treatment. J. Membr. Sci. 1997, 123, 95-103. (3) Tschaufeser, P.; Parker, S. Thermal Expansion Behavior of Zeolites and ALPO4s. J. Phys. Chem. 1995, 99, 10609-106015. (4) den Exter, M. J.; van Bekkum, H.; Rijn, C. J. M.; Kapteijn, F.; Moulijn, J. A.; Schellevis, H.; Beenakker, C. I. N. Stability of oriented silicalite-1 films in view of zeolite membrane preparation. Zeolites 1997, 19, 13-20. (5) Dong, J.; Lin, Y. S.; Hu, M. Z.-C.; Peascoe, R. A.; Payzant, E. A. Template-removal-associated microstructural development of porous-ceramic-supported MFI zeolite membranes. Microporous Mesoporous Mater. 2000, 34, 241-253. (6) Geus, E. R.; van Bekkum, H. Calcination of large MFI-type single crystals. Part 2: Crack formation and thermomechanical properties in view of the preparation of zeolite membranes. Zeolites 1995, 15, 333-341. (7) Beck, L. W.; Davis, M. E. Alkylammonium Polycations as Structure-Directing Agents in MFI Zeolite Synthesis. Microporous Mesoporous Mater. 1998, 22, 107-114. (8) Goretsky, A. V. L. W. B.; Zones, S.; Davis, M. E. Influence of the hydrophobic character of structure-directing agents for the synthesis of pure-silica zeolites. Microporous Mesoporous Mater. 1999, 28, 387-393. (9) Mintova, S.; Valtchev, V.; Vulcheva, E.; Veleva, S. Kinetics of Zeolite ZSM-5 Crystallization: Template Effect of PropylSubstituted Amines. Mater. Res. Bull. 1992, 27, 515-522. (10) Lee, J.-C.; Hayhurst, D. H. Evaluation of Reaction Kinetics for the Decomposition of Tetrapropylammomium Cations in Silicalite. Chem. Eng. Commun. 1989, 77, 15-23. (11) El Hage-Al Asswad, J.; Dewaele, N.; Nagy, J. B.; Hubert, R. A.; Gabelica, Z.; Derouane, E. G.; Crea, F.; Aiello, R.; Nastro,
Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 4849 A. Identification of different tetrapropylammonium cations occluded in ZSM-5 zeolite by combined thermal analysis and NMR spectroscopy. Zeolites 1988, 8, 221. (12) Parker, L.; Bibby, B. M.; Patterson, J. E. Thermal Decomposition of ZSM-5 and silicalite precursors. Zeolites 1984, 4, 168174. (13) Soulard, M. S. B.; Kessler, H.; Guth, J. L. Characterization of the products remaining in the solid after partial thermal decomposition of Pr4NF-, Pr3NHF-, and Pr4NOH-MFI precursors. Zeolites 1991, 11, 107-115. (14) Bilger, S.; Soulard, M.; Kessler, H.; Guth, J. L. Identification of the volatile products resulting from the themal decomposition of tetra-, tri-, di- and mono-n-propylannomium cations occluded in the MFI-type zeolites. Zeolites 1991, 11, 784-791. (15) Nowotny, M.; Lerchern, J. A.; Kessler, H. IR Spectroscopy of Single Zeolite Crystals Part 1: Thermal Decompostiion of the Template in MFI-type materials. Zeolites 1991, 11, 454-459. (16) Hurst, K. E. Template Decomposition in Silicalite. M.S. Thesis, Colorado School of Mines, Golden, CO, 2000; p 113.
(17) Petersen, J.; Peinemann, K. V. Preparation and gas permeation properties of ceramic-silicalite membranes. J. Mater. Sci. Lett 1996, 1777-1780. (18) Jacobs, P. A.; Martens, J. A. Synthesis of ZSM-5 with TPA. In Synthesis of High-Silica Aluminosilicate Zeolites; Elsevier Science: Amsterdam, 1987. (19) Derouane, E.; Detremmrie, S.; Gabelica, Z.; Blom, N. Synthesis and characterization of ZSM-5 Type Zeolites 1. Physicochemical Properties of Precursors and Intermediates. Appl. Catal. 1981, 1, 201-224. (20) Soulard, M.; Bilger, S.; Kessler, H.; Guth, J. L. Thermoanalytical characterization of MFI-type zeolites perepared either in the presence of OH- or of F- ions. Zeolites 1987, 7, 463-470.
Received for review January 22, 2001 Revised manuscript received July 16, 2001 Accepted June 26, 2001 IE010069V
