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Enhancement of n-Hexane Cracking Activity over Modified ZSM-12 Zeolites Kye Sang Yoo,*,† Srikant Gopal,‡ and Panagiotis G. Smirniotis§ Environment and Process Technology Division, Korea Institute of Science and Technology, Seoul 130-791, South Korea, SABIC Research & Technology Private Ltd., Baroda, India, and Department of Chemical Engineering, University of Cincinnati, Cincinnati, Ohio 45221
A series of ZSM-12 samples were prepared by calcining them at different temperatures that are much higher than what is normally used. The catalytic activities of these samples were tested using cracking of n-hexane at 400 °C. It was found that the cracking activities of samples calcined at certain temperatures were unusually high: 3-4 times greater than that of the regular sample, which was calcined at 550 °C. Interestingly, this enhanced activity was observed at two separate, relatively narrow, ranges of temperatures: one around 650 °C and the other around 800 °C. Bro¨nsted and Lewis acid sites attributed to extraframework cationic aluminum were quantified with Fourier transform infrared of adsorbed pyridine. The Lewis/Bro¨nsted ratios of the enhancedactivity samples were high compared those of the low-activity samples; the trend in variation of the Lewis/Bro¨nsted ratios of the samples with calcination temperature closely mirrored the trend observed for the catalytic activities. This is an indication that there is a synergistic interaction between the Bro¨nsted and Lewis acid sites that is responsible for the enhanced cracking activity. 1. Introduction Zeolites are rarely used as acid catalysts in the synthesized form; they are usually subjected to treatments that modify their structural and acidity characteristics, a process sometimes referred to as activation.1,2 Dealumination of zeolites by contacting them with steam at high temperatures is a widely used treatment applied to many zeolite catalysts before they are used commercially. This steam treatment is known to beneficially affect several properties of the zeolite, improving thermal and hydrothermal stability, enhancing catalytic activity, generating mesopores that help in faster diffusion of reactant and product molecules, and in certain cases decreasing the coking severity over the catalyst.3-5 There is a very vast amount of literature particularly investigating the reasons for the enhanced activity of zeolites after steam treatments. However, even though there is a much better understanding of these zeolite catalysts now, the exact chemical nature of the enhanced-activity sites is still unresolved and under debate.3 ZSM-12 is a one-dimensional large-pore zeolite with pore openings of 5.6 × 6.0 Å. In our recent work, we have detailed the beneficial properties of ZSM12 in various reactions.6-8 Because of its smaller pore size, compared to other typical large-pore zeolites, ZSM12 shows good coking resistance even under severe conditions and gives a higher yield of the desired branched isomers in hydroisomerization reactions.8,9 However, because of the one-dimensional nature of its pores, ZSM-12 shows a poorer activity compared to β-zeolite, which is three-dimensional; in hydroisomerization reactions, ZSM-12 requires a significantly higher temperature than β-zeolite to reach a certain conversion level.8 Our objective in this study was to subject ZSM* To whom correspondence should be addressed. Tel.: +822-958-5871. Fax: +82-2-958-5809. E-mail:
[email protected]. † Korea Institute of Science and Technology. ‡ SABIC Research & Technology Private Ltd. § University of Cincinnati.
12 to various steam and thermal treatments and to try to enhance its activity. A conventional steaming treatment proved to be too severe and caused extensive dealumination; however, a novel approach, which involved calcination of the synthesized zeolite (to remove the template) at temperatures higher than what is normally used, yielded positive results. Moreover, a very interesting trend in the hexane cracking activity of the samples with respect to the calcination temperature was obtained. This unusual trend, combined with characterizations, provided clues to explain the reason for the enhanced catalytic activity of ZSM-12. 2. Experimental Section 2.1. Zeolite Synthesis and Modification. Pure, highly crystalline ZSM-12 with a Si/Al ratio of 35 was synthesized hydrothermally using tetraethylammonium hydroxide (TEAOH) as well as tetraethylammonium bromide (TEABr) as the template.10,11 The synthesized zeolite was recovered by filtration, washed thoroughly with deionized water, and dried at 120 °C. Normally, removal of the template from ZSM-12 is accomplished by calcination in air at a temperature of 550 °C. In this study, apart from a sample calcined at 550 °C, a series of samples (referred to as modified samples in this paper) were prepared by calcination of the zeolite at temperatures ranging from 600 to 900 °C. Calcination of the ZSM-12 samples was carried out in a 2-cmdiameter quartz reactor, using 1 g of the zeolite, under a 50-cm3/min air flow. The air was provided from a cylinder (Wright Brothers), and the flow was maintained constant using a mass flow controller (MKS). The heating rate during the calcination was 6 °C/min, and after the sample reached the designated temperature, it was held at that temperature for 2 h. The calcination temperature was accurately controlled by positioning the thermocouple in the center of the zeolite bed, which was about 1.5 cm deep. After calcination, the zeolites were converted into the ammonium forms by cation
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exchange with a 2 N NH4Cl solution at 90 °C for 4 h, filtered, and dried. 2.2. Catalytic Reaction. The catalytic activities of the modified ZSM-12 samples were tested using nhexane cracking as a probe reaction. The experiments were carried in a flow reactor system incorporating a 1/ -in.-o.d. stainless steel reactor, with the catalyst 4 loaded on top of a quartz wool plug. Pretreatment and protonation of the ammonium form of the zeolite was done in situ by heating the sample at 500 °C under a nitrogen flow (30 mL/min) for 1 h and then cooling it to 400 °C. The n-hexane cracking reaction was carried out at 400 °C with 25 mg of the zeolite (diluted with 40 mg of silica). The nitrogen flow rate was 30 mL/min, and n-hexane was fed to the reactor at a rate of 2 mL/min through a septum injection port using a liquid infusion pump. Identification of the products was accomplished using a gas chromatograph (Hewlett-Packard, 5890 series II) equipped with a mass spectrometer (HewlettPackard, 5972 series II); separation was achieved using a capillary column (Supleco Petrocol DH50.2). The concentration of the products was calculated by using a calibration factor for each product. n-Hexane conversion was given by (1 - unreacted hexane/hexane fed) × 100. The n-hexane conversion was less than 10% over all of the samples; the reaction rates were calculated from the rate expression for differential conditions:12 Fx ) rW, where F is the flow rate of the reactant (mol/s), x the conversion, r the reaction rate (mol g-1 s-1), and W the weight of the catalyst (g). Because the reactions with all of the samples were carried out under identical conditions using the exact same amount of catalyst, the relative activity can be obtained just by dividing the conversion over a particular sample by the conversion over a reference sample. In this study, we have used the n-hexane conversion over the sample calcined at 550 °C as the “parent” or reference sample; the relative activities discussed later are with respect to this sample. 2.3. Characterization. X-ray diffraction (XRD) was employed for ascertaining the purity and high crystallinity of the synthesized ZSM-12 and quantification of the crystallinities of the modified samples. The XRD patterns were collected with a Siemens powder X-ray diffractometer using Cu KR radiation. The relative crystallinity of the modified zeolites was estimated based on the heights of the main crystallographic peaks of ZSM-12, with the parent sample as the reference. The Brunauer-Emmett-Teller (BET) specific surface areas of the zeolites (used as a measure of the nitrogen filling capacity of the samples) were measured by nitrogen adsorption at 77 K using a Micromeritics Gemini 2360 instrument. All samples were degassed with helium for 2 h at 250 °C prior to the measurement. Scanning electron microscopy (SEM) was done on selected samples to determine the crystallite size and morphology using a Hitachi 2700 scanning electron microscope. Nitrogen adsorption isotherms were measured for selected samples at 77 K using a Micromeritics ASAP 2000 instrument; samples were degassed at 250 °C for 2 h. Acidity characteristics of the parent and modified samples were investigated using IR spectroscopy. Fourier transform infrared (FTIR) measurements were performed on a BioRad FTS-40 spectrometer equipped with a high-temperature flow cell having CaF2 windows. The numbers of Bro¨nsted and Lewis acid sites were determined by IR spectroscopy of adsorbed pyridine. Pyridine was adsorbed on the sample by injecting it into the gas line
Figure 1. XRD patterns of the sample calcined at various temperatures.
through a septum injection port. Injection of pyridine at a rate of 0.2 mL/min for 20 min was found to be enough to completely saturate the sample. The sample was purged with helium for 1 h to remove the physisorbed pyridine, and a spectrum was collected. The temperature was then increased to 200 °C, held for 1 h to desorb more pyridine, and decreased again to 150 °C, and a spectrum was recorded. This stepwise desorption procedure was repeated for two more desorption temperatures, 300 and 400 °C; the spectra were always recorded at 150 °C. X-ray photoelectron spectroscopy (XPS) was used to analyze the surface atomic concentrations of the elements in selected modified samples. The spectra were recorded on a Perkin-Elmer model 5300 using Mg KR (1253.6 eV) as a radiation source at 300 W. 3. Result and Discussion 3.1. Physical Properties. XRD of the modified samples indicated that the zeolite crystallinity was maintained to a large extent even after calcination at 900 °C. Up to a calcination temperature of 800 °C, the crystallinity of the modified samples relative to the parent sample was always greater than 90%. Considering the possible error in this method, we can say that there was hardly any decrease in the zeolite crystallinity. Only the sample calcined at 850 °C showed a slight drop in crystallinity, with its crystallinity being about 85%, as shown in Figure 1. Physical properties of the samples also showed the same trend: parent and modified samples up to a calcination temperature of 800 °C showing similar surface areas and the sample calcined at 850 °C showing a slightly lower surface area. These properties of the samples that were analyzed are given in Table 1. 3.2. n-Hexane Cracking. The change of conversion with time on stream is illustrated in Figure 2. The catalytic activity was very stable regardless of the samples. This is mainly attributed to the mild reaction conditions to allow us to better monitor the catalytic activity without deactivation. The relative activities of the samples were determined by carrying out n-hexane cracking at 400 °C. When these were plotted against the calcination temperature (see Figure 3), a very interesting trend is observed. The samples calcined at temperatures of around 650 and 800 °C show a dra-
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Table 1. Characterizations of Selected ZSM-12 Samples Calcined at Different Temperatures
sample
BET surface area (m2/g)
external surface area (m2/g)
micropore volumea (cm3/g)
mesopore volumeb (cm3/g)
Z550 Z650 Z775 Z800 Z850
297 291 286 287 289
75 77 75 76 70
0.15 0.14 0.14 0.14 0.13
0.25 0.25 0.24 0.24 0.24
a Determined by the t-plot method. b Determined by the Barrett-Joyner-Halenda method.
Figure 2. Change of the n-hexane conversion with respect to time on stream over various samples. The reaction was performed at 400 °C.
Figure 3. Relative n-hexane cracking activity of the ZSM-12 samples plotted against the temperature at which they were calcined. The activity of the sample calcined at 550 °C has been set at unity, and the activities of the other samples are relative to this.
matically higher activity, 3-4 times higher than that of the sample calcined at 550 °C. Interestingly, these “maxima” occur only in a somewhat narrow range of temperatures. In the first maximum, only the Z650 and Z675 samples show a high activity, and a further 25 °C increase in temperature causes the activity of the Z700 sample to drop to a value below that of the parent sample. For the second maximum, the temperature range is somewhat broader; Z775, Z800, and Z825 all show high activities. The samples to the left and right of the second maximum, Z750 and Z850, respectively, have activities lower than that of the parent sample.
Figure 4. FTIR spectra of the O-H stretching region of the ZSM12 samples calcined at various temperatures: (a) Z550; (b) Z600; (c) Z650; (d) Z700; (e) Z750; (f) Z800; (g) Z850. The spectra have been offset for clarity.
In the literature, it is well-known that steam dealumination can result in acid sites with dramatically enhanced activity in low silica zeolites.3,4 There are also several reports on the enhancement in catalytic activities of high silica zeolites such as ZSM-5 after steaming under mild conditions.2,13,14 In our work, it appears that the combustion of the template retained in the zeolite pores caused conditions similar to mild steam dealumination, and some dehydroxylation or “self-steaming” also probably occurred at the proton sites left behind after removal of the template. The presence of the first maximum at around 650 °C could be explained based on the results available in the literature because other studies have also observed a maximum in the catalytic activity with increasing severity of the treatment.13 However, the appearance of a second maximum with a further increase in the calcination temperature was very baffling. Therefore, in an effort to understand the exact reason(s) and provide an explanation for the specific trend in the catalytic activity that we observed with the calcination temperature, we performed several characterizations, the results of which are discussed in the following sections. 3.3. FTIR of the Hydroxyl Region. The FTIR spectra of the OH stretching region of the ZSM-12 samples calcined at different temperatures are shown in Figure 4. Because of the high bulk Si/Al ratio (Si/Al ∼ 30) of the samples, the intensities of various bands are quite low and it is difficult to observe very obvious differences. Nevertheless, upon close inspection, the spectra do show some differences. The spectrum of the sample calcined at 550 °C shows four broad maxima; on the basis of the literature, we attempted to assign these bands to the various hydroxy groups. Usually for zeolites, a prominent and relatively sharp maximum at
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3745 cm-1, attributed to terminal silanols located at the external surface, is observed;15,16 however, in our ZSM12 samples, a maximum at this wavenumber is not observed, although it is possible that it could be hidden. Instead, a maximum at about 3732 cm-1 is observed; Busca and co-workers16 and Guisnet et al.17 observed a shoulder at a wavenumber of 3735 cm-1, and Busca assigned this to silanol groups inside the zeolite pore. This assignment seems to make sense because the intensity of this maximum decreases with an increase in the calcination temperature, consistent with dehydroxylation and some structure destruction at higher temperatures. The broad band with a maximum at 3667 cm-1 has been assigned by many researchers to AlOH groups, with the aluminum atoms in the extraframework position or partially linked to the framework.15,18,19 For the samples calcined at 700 °C and higher, an increase in the intensity of this band is observed, clearly indicating the dislodgement of aluminum from the framework. The other band that is of significance is the very broad band ranging from 3625 to about 3400 cm-1. According to Kustov,15 most authors tend to ascribe this band to hydrogen-bonded hydroxyls or traces of water, but they definitely are characteristics of acidity. Others16,18 also state that this band can be associated with H-bonded silanols present in defect positions or to Si(OH)Al groups interacting via H bonds with neighboring oxygen. In many zeolites, the Bro¨nsted Si(OH)Al peak is observed at around 3610 cm-1; however, this peak is absent or hidden in our ZSM-12 samples. Paze et al.20 have assigned the broad band in the 3600-3200-cm-1 range to bridged hydroxyls that are perturbed by H-bond interactions with the zeolite framework. Therefore, it appears that the broad band in the 3625-3400-cm-1 range in Figure 4 can be attributed mainly to the Bro¨nsted Si(OH)Al groups. The gradual decrease in the intensity of this band with an increase in the calcination temperature agrees with this assignment and explains the changes in the samples. Although some information could be obtained, the data from the hydroxyl region do not provide enough information to explain the trend observed in the catalytic activities of the samples with the calcination temperature. 3.4. FTIR of Adsorbed Pyridine. Pyridine adsorbed on an acidic zeolite has been widely used to characterize and quantify the Bro¨nsted and Lewis acid sites in a zeolite. The FTIR spectra of pyridine adsorbed on the various ZSM-12 samples are shown in Figure 5. The spectra have been normalized based on the weight of the wafer used in the experiment and have been deliberately offset in Figure 5 for clarity. Several typical peaks associated with the interaction of pyridine with the acid sites are observed. The bands at 1545 and 1637 cm-1 are associated with Bro¨nsted acid sites and assigned to the pyridinium ion (PyH+); the two bands at 1455 and 1622 cm-1 are related to Lewis bonded pyridine; the peak at 1490 cm-1 is due to the pyridine molecule itself and therefore can be considered as the superposition of signals from pyridine adsorbed on both Bro¨nsted and Lewis acid sites.18 The peak at 1445 cm-1 has typically been attributed to physisorbed pyridine and has been claimed to disappear after desorption at 200 °C21 or even at 150 °C.22 However, in our case, this peak is observed even at 200 °C (see Figure 5) but decreased with a further increase in the temperature and almost completely disappeared in many samples
Figure 5. FTIR spectra of pyridine adsorbed on the ZSM-12 samples calcined at various temperatures. This figure shows pyridine remaining in the zeolite after desorption at 200 °C. The spectra have been offset for clarity.
after desorption at 400 °C (not shown). It is, therefore, unlikely that this peak is entirely due to physisorbed pyridine; in fact, Marques et al.18 suggest that the bands at 1445 and 1602 cm-1 correspond to pyridine adsorbed on a new type of Lewis acid site. The most important bands, however, are the one at 1545 cm-1 related to Bro¨nsted acid sites and the one at 1455 cm-1 related to Lewis acid sites. More specifically, the band at 1455 cm-1 has been attributed to cationic Al3+ species.21 It is possible to determine the concentrations of the Bro¨nsted and Lewis acid sites from the area of these bands; extinction coefficients for each band are available in the literature for this purpose. The extinction coefficients provided by Emeis22 have been widely used in the literature; therefore, we decided to use this to determine the concentrations of the accessible Bro¨nsted and Lewis acid sites on the ZSM-12 samples. In Figure 5, the peaks at 1455 and 1445 cm-1 are not completely separated; therefore, a deconvolution procedure was used to separate the peaks and determine the area of the 1455 cm-1 peak more accurately (see Figure 6). The concentration of the Bro¨nsted and Lewis acid sites (1455 cm-1) and the Lewis/Bro¨nsted ratio have also been plotted against the calcination temperature in Figure 7, which makes the trend in the variation of the numbers of Bro¨nsted and Lewis acid sites with the calcination temperature easier to observe. It can be seen that the Bro¨nsted acidity decreases gradually with an increase in the calcination temperature up to 650 °C but then appears to reach a plateau and is almost constant up to a temperature of 750 °C. With a further increase in the calcination temperature, the concentration of Bro¨nsted acid sites rapidly decreases by about 50%, but there is no further decrease with an increase in the calcination temperature, and another plateau
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Figure 6. Deconvolution of FTIR spectra of Lewis sites region. This figure shows pyridine remaining in the zeolite after desorption at 200 °C. Table 2. Amount of Charge Compensating for TEA+ and Na+ Cations in ZSM-12 Samples of Different Si/Al Ratios Synthesized Using TEABr bulk Si/Al ratio of synthesized zeolite
Al/uc
TEA+/uc
Na/uc
(Al-TEA+)/uc
30 38 48 56 98
0.90 0.72 0.57 0.49 0.28
0.47 0.40 0.19 0.15 0.00
0.54 0.44 0.85 0.93 0.87
0.43 0.32 0.38 0.34 0.28
appears. To explain why the concentration of Bro¨nsted acid sites does not decrease further after 775 °C, it must be remembered that in this study the as-synthesized zeolites were directly calcined at the higher temperatures to produce the modified samples. In our studies on the synthesis of ZSM-12,10,11 we determined the amounts of Na+ and tetraethylammonium (TEA+) cations compensating for the charge of the zeolite framework by thermogravimetric analysis. Some of the relevant results from that study are given in Table 2. From this table, it can be estimated that a ZSM-12 with a Si/ Al ratio of 35 will have about 55% of its framework charge compensated for by the TEA+ cation. Only these sites can be converted into proton sites upon removal of the template. Because zeolites can be hydrothermally dealuminated and can undergo dehydroxylation only in their proton form,1 the remainder of the sites whose charge is compensated for by Na+ must remain intact. This is in excellent agreement with the trend in Figure
7 and explains why aluminum remains in the framework even at such high temperatures. Meanwhile, in Figure 7, the concentration of Lewis acid sites increases with an increase in the calcination temperature up to 650 °C, then drops by 50% with a further increase in the temperature and stays fairly constant (it should be noted that the scale for the Lewis acid concentration is greatly magnified), and starts decreasing again above 800 °C. The reason for the decrease in the Lewis acid site concentration is not very clear. It is possible that some of them are converted into the other type of Lewis acid sites mentioned earlier or, if the Lewis acidity is really due to cationic aluminum, then its agglomeration with an increase in the temperature may be the reason. The most interesting set of data from Figure 7 is the variation of the Lewis/ Bro¨nsted ratio with the calcination temperature. It is astonishing to note that the trend in the variation of the Lewis/Bro¨nsted ratio in Figure 7 almost mirrors the trend in the variation of the hexane cracking activity with calcination temperature (Figure 3). This is an indication that the reason for the enhanced activity of the samples calcined around 650 and 800 °C is due to a synergistic interaction between the Bro¨nsted and Lewis acid (extraframework aluminum, EFAL) sites. Several research groups have suggested that the synergistic interaction between the EFAL and the Bro¨nsted acid sites is the reason for the enhanced activity observed in steam-dealuminated zeolites.3,4,23 It can be said by looking at Figure 7 that the Lewis acid sites themselves are the reason for the enhanced activity. However, if this were the case, the sample calcined at 750 °C should show a high activity, but its activity is actually lower than that of the reference sample (Figure 3). However, when the Bro¨nsted acidity sharply decreases upon a temperature increase from 750 to 775 °C, the Lewis/ Bro¨nsted ratio again reaches a high value and the catalysts show enhanced activity. To check if any other reasons such as the creation of mesoporosity or surface enrichment of aluminum were responsible for enhancement of the activity, nitrogen adsorption isotherms, SEM, and XPS experiments were performed. For the nitrogen adsorption experiments, a different set of ZSM12 samples synthesized using TEAOH were used. It
Figure 7. Variation of the concentration of Bro¨nsted acid sites (b), Lewis acid sites (9), and the Lewis/Bro¨nsted (L/B) ratio (O) for the ZSM-12 samples with calcination temperature.
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4. Conclusions Calcination of ZSM-12 to remove the template, at certain temperatures higher than what is commonly used, yielded samples with hexane cracking activities 3-4 times greater than that of the regular sample calcined at 550 °C. A very unusual trend in the variation of the catalytic activity with the calcination temperature was observed; only samples calcined at two narrow ranges of temperatures, one around 650 °C and the other around 800 °C, showed enhanced activity. FTIR characterizations of the samples with pyridine adsorbed on the acid sites allowed quantification of Bro¨nsted acid sites and Lewis acidic EFAL, possibly present as a cationic Al3+ species. The FTIR data indicated that samples with enhanced activity had a high ratio of the Lewis/Bro¨nsted acid sites. The trend in the variation of the Lewis/Bro¨nsted sites ratio of the samples with an increase in the calcination temperature was almost identical with the trend observed in the variation of the catalytic activity. This excellent agreement of the FTIR characterization data with the catalytic activity of the samples clearly proved that the enhancement of the catalytic activity of certain samples was due to a synergistic interaction between the Bro¨nsted acid sites and Lewis acidic EFAL species. The occurrence of enhanced catalytic activities at two separate temperature ranges due to the use of as-synthesized samples allowed clearer identification of synergism between the Bro¨nsted and Lewis acid sites as the reason for the activity enhancement. Figure 8. SEM micrographs of the ZSM-12 samples calcined at 550 °C (a) and 850 °C (b). The scale on the top picture is 1 µm, and the bottom picture is 500 nm.
should be mentioned here that, to confirm the veracity of the catalytic results, the hexane cracking experiments were performed with three different sets of samples (all with a Si/Al ratio of 30) and very similar trends in the relative catalytic activity were obtained. The SEM and XPS experiments were performed on the same set of samples as those used to perform the catalytic tests and FTIR characterizations reported in this paper. XPS showed that that the surface Si and Al concentrations were the same for the Z550, Z650, and Z800 samples; there was no change at all in the surface aluminum concentration even after calcination at such a high temperature. SEM micrographs of the Z550 and Z850 samples are shown in Figure 8, and the external surface areas and pore volumes of the samples studied by nitrogen adsorption are summarized in Table 1. Although the SEM micrographs show some reduction in the zeolite crystal size after calcination at 850 °C and some indication of the presence of cracks in the crystal, there is hardly any difference between the samples in terms of the external surface area and pore volumes (Table 1). This suggests that, even though there still is a possibility that the presence of cracks or mesopores could have caused the enhancement in the catalytic activity of the Z775, Z800, and Z825 samples (second maximum in Figure 3), their contribution to the activity increase alone would not be sufficient to explain the dramatically high activities of these samples. The hypothesis that can explain the enhanced activity of certain samples in this study is the synergistic interaction between the Bro¨nsted acis sites and the extraframework Lewis acid sites.
Literature Cited (1) Beyer, H. K. Dealumination techniques for zeolites. In Molecular sieves Science and Technology. Postsynthesis modification I; Karge, H. G., Weitkamp, J., Eds.; Springer-Verlag: Berlin, 2002; Vol. 3. (2) Hutchings, G. J.; Burrows, A.; Rhodes, C.; Kiely, C. J.; McClung, R. Dealumination of mordenite catalysts using a low concentration of steam. J. Chem. Soc., Faraday Trans. 1997, 93, 3593. (3) Beyerlein, R. A.; ChoiFeng, C.; Hall, J. B.; Huggins, B. J.; Ray, G. J. Effect of steaming on the defect structure and acid catalysis of protonated zeolites. Top. Catal. 1997, 4, 27. (4) Remy, M. J.; Stanica, D.; Poncelet, G.; Feijen, E. J. P.; Grobet, P. J.; Martens, J. A.; Jacobs, P. A. Dealuminated H-Y zeolites: Relation between physicochemical properties and catalytic activity in heptane and decane isomerization. J. Phys. Chem. 1996, 100, 12440. (5) Kung, H. H.; Williams, B. A.; Babitz, S. M.; Miller, J. T.; Haag, W. O.; Snurr, R. Q. Enhanced hydrocarbon cracking activity of Y zeolites. Top. Catal. 2000, 10, 59. (6) Zhang, W.; Smirniotis, P. G. On the exceptional time-onstream stability of HZSM-12 zeolite: relation between zeolite pore structure and activity. Catal. Lett. 1999, 60, 223. (7) Zhang, W.; Smirniotis, P. G. Effect of zeolite structure and acidity on the product selectivity and reaction mechanism for n-octane hydroisomerization and hydrocracking. J. Catal. 1999, 182, 400. (8) Gopal, S.; Smirniotis, P. G. Pt/H-ZSM-12 as a catalyst for the hydroisomerization of C5-C7 n-alkanes and simultaneous saturation of benzene. Appl. Catal. A 2003, 247, 113. (9) Gopal, S.; Smirniotis, P. G. Deactivation behavior of bifunctional Pt/H-zeolite catalysts during cyclopentane hydroconversion. J. Catal. 2002, 205, 231. (10) Gopal, S.; Yoo, K.; Smirniotis, P. G. Synthesis of Al-rich ZSM-12 using TEAOH as template. Microporous Mesoporous Mater. 2001, 49, 149. (11) Yoo, K.; Kashfi, R.; Gopal, S.; Smirniotis, P. G.; Gangoda, M.; Bose, R. N. TEABr directed synthesis of ZSM-12 and its NMR characterization. Microporous Mesoporous Mater. 2003, 60, 57. (12) Jacobs, P. A.; Martens, J. A. Introduction to acid catalysis with zeolites in hydrocarbon reactions. In Introduction to zeolites
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science and practice; Bekkum, H. V., Flanigen, E. M., Jansen, J. C., Eds.; Elsevier: Amsterdam, The Netherlands, 1991; Vol. 58, pp 445-496. (13) Lago, R. M.; Haag, W. O.; Mikovsky, R. J.; Olson, D. H.; Hellring, S. D.; Schmitt, K. D.; Kerr, G. T. The nature of catalytic sites in HZSM-5sActivity Enhancement. Stud. Surf. Sci. Catal. 1986, 28, 677. (14) Datka, J.; Marschmeyer, S.; Neubauer, T.; Meusinger, J.; Papp, H.; Schutze, F. W.; Szpyt, I. Physicochemical and catalytic properties of HZSM-5 zeolites dealuminated by the treatment with steam. J. Phys. Chem. 1996, 100, 14451. (15) Kustov, L. M. New trends in IR-spectroscopic characterization of acid and basic sites in zeolites and oxide catalysts. Top. Catal. 1997, 4, 131. (16) Trombetta, M.; Busca, G.; Storaro, L.; Lenarda, M.; Casagrande, M.; Zambon, A. Surface acidity modifications induced by thermal treatments and acid leaching on microcrystalline H-BEA zeolite. A FTIR, XRD and MAS-NMR study. Phys. Chem. Chem. Phys. 2000, 2, 3529. (17) Guisnet, M.; Ayrault, P.; Datka, J. Acid properties of mazzite zeolites studied by IR spectroscopy. Microporous Mesoporous Mater. 1998, 20, 283. (18) Marques, J. P.; Gener, I.; Ayrault, P.; Bordado, J. C.; Lopes, J. M.; Ribeiro, F. R.; Guisnet, M. Infrared spectroscopic study of the acid properties of dealuminated BEA zeolites. Microporous Mesoporous Mater. 2003, 60, 251.
(19) Chiche, B. H.; Dutartre, R.; Direnzo, F.; Fajula, F.; Katovic, A.; Regina, A.; Giordano, G. Study of the Sorption and Acidic Properties of Mtw-Type Zeolite. Catal. Lett. 1995, 31, 359. (20) Paze, C.; Zecchina, A.; Spera, S.; Cosma, A.; Merlo, E.; Spano, G.; Girotti, G. Comparative IR and 1H MAS NMR study of adsorption of CD3CN on zeolite H-beta: evidence of the presence of two families of bridged Bronsted sites. Phys. Chem. Chem. Phys. 1999, 1, 2627. (21) van Bokhoven, J. A.; Tromp, M.; Koningsberger, D. C.; Miller, J. T.; Pieterse, J. A. Z.; Lercher, J. A.; Williams, B. A.; Kung, H. H. An explanation for the enhanced activity for light alkane conversion in mildly steam dealuminated mordenite: The dominant role of adsorption. J. Catal. 2001, 202, 129. (22) Emeis, C. A. Determination of Integrated Molar Extinction Coefficients for Infrared-Absorption Bands of Pyridine Adsorbed on Solid Acid Catalysts. J. Catal. 1993, 141, 347. (23) Hong, Y.; Gruver, V.; Fripiat, J. J. Role of Lewis Acidity in the Isomerization of n-Pentane and o- Xylene on Dealuminated H-Mordenites. J. Catal. 1994, 150, 421.
Received for review November 25, 2004 Revised manuscript received April 20, 2005 Accepted May 2, 2005 IE0488602
