Dehydrogenation of Propane over Platinum Containing CIT-6

is contacted with NH4NO3 to remove the SDA prior to platinum loading, ... When as-made CIT-6 is contacted with acetic acid at 60 °C, the SDA and also...
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Ind. Eng. Chem. Res. 2004, 43, 2922-2928

Dehydrogenation of Propane over Platinum Containing CIT-6 P. Andy and M. E. Davis* Chemical Engineering, California Institute of Technology, Pasadena, California 91125

The catalytic dehydrogenation of propane is carried out on platinum-impregnated CIT-6 catalysts. Prior to loading platinum, numerous postsynthesis treatments of the as-made CIT-6 are investigated, e.g., calcination, structure directing agent (SDA) extraction, ion exchange, etc., to observe the effects of catalyst preparation on the dehydrogenation of propane. The catalytic behavior depends strongly on the method of postsynthesis treatment. When the as-made CIT-6 is contacted with NH4NO3 to remove the SDA prior to platinum loading, an active, selective, and stable catalyst is obtained. 29Si NMR studies show that, after this postsynthesis treatment, some of the zinc remains within the CIT-6 framework. Contacting as-made CIT-6 with Zn(NO3)2 to remove the SDA prior to platinum loading gives a solid with a higher level of extraframework zinc. This additional amount of extraframework zinc does not lead to a more stable catalyst. When as-made CIT-6 is contacted with acetic acid at 60 °C, the SDA and also the zinc are extracted to give pure-silica CIT-6. Platinum loading into this material gives a catalyst that deactivates very rapidly. Thus, framework zinc appears to be essential for the creation of a stable platinum-impregnated CIT-6 catalyst for propane dehydrogenation. 1. Introduction The dehydrogenation of propane into propylene is an important reaction from the industrial point of view.1-3 Indeed, propylene is a raw material for numerous products such as polypropylene and acrylonitrile. The dehydrogenation of propane is a thermodynamically limited reaction that operates at high temperature. Because of this issue, low conversions and catalyst deactivation due to coke formation are typically observed.4 Two types of catalysts have been developed for the catalytic dehydrogenation of light olefins: chromiumbased catalysts and platinum-supported catalysts.2 With the platinum-containing materials, the addition of tin or zinc promoters is used to maintain catalyst activity.2,5-7 Workers at British Petroleum (BP) have reported the dehydrogenation of isobutane using a catalyst that contained platinum impregnated into a zincosilicate molecular sieve of ZSM-5 type (we will denote this type of material as Pt/Zn-MFI). This catalyst possessed a high dispersion of the active phase (thought to be a Pt-Zn alloy) and gives high activity and selectivity for alkane dehydrogenation. Moreover, the catalyst is highly resistant to coke formation and thus reveals good stability.2,8 Our group recently synthesized a new large-pore zincosilicate denoted as CIT-6.9,10 CIT-6 is the first molecular sieve to contain zinc in framework sites that are accessible to organic molecules. Moreover, the zinc can be partially or totally extracted from the CIT-6 framework depending on the postsynthesis treatments carried out.10 When the structure directing agent (SDA) is partially extracted by contacting CIT-6 with NH4NO3, the zinc stays within the CIT-6 framework. However, in the absence of this exchange, calcination in air leaves only a portion of the zinc in framework positions. Alternatively, an extraction of the SDA with acetic acid * To whom correspondence should be addressed. Tel.: 626-395-4251. Fax: 626-568-8743. E-mail: mdavis@ cheme.caltech.edu.

leads to the removal of all of the zinc and gives puresilica CIT-6. Based on the findings from workers at BP with Pt/ Zn-MFI, platinum-impregnated CIT-6 may be an interesting catalyst for the dehydrogenation of propane. The ability to use postsynthesis treatments to give different compositions of CIT-6 with the same framework structure may allow for elucidation of the role of zinc in the dehydrogenation of propane into propylene. The goal of this work is to prepare and test Pt/CIT-6 as a catalyst for the dehydrogenation of propane and to compare the results obtained to those from a BP-type catalyst. 2. Experimental Section 2.1. Preparation of Catalysts. 2.1.1. Zincosilicate-MFI. Chemical reagents used were zinc sulfate heptahydrate (EM Science), ammonia solution (26% in water, Mallinckrodt), sodium hydroxide (EM Science), tetrapropylammonium hydroxide (TPAOH; 1.0 M aqueous solution, Aldrich), and colloidal silica (Ludox AS 40, Aldrich). Zincosilicate-MFI was synthesized using the method reported by BP.8 Zinc sulfate was dissolved in distilled water. An ammonia solution was added to increase the pH to 6. The precipitate formed was filtered and washed thoroughly with distilled water. Sodium hydroxide dissolved in distilled water was added to the precipitate with stirring. TPAOH was added to the mixture followed by Ludox AS 40 addition with thorough stirring to maintain a homogeneous reaction mixture. The molar composition of the mixture was 1.6: 1.58:15.1:1:218.7 Na2O/TPAOH/SiO2/ZnO/H2O. This reaction mixture was charged into a Teflon-lined autoclave and heated in rotary mode under autogenous pressure at 175 °C for 4 days. The sample was recovered by filtration, washed with water, and dried overnight in air at 100 °C. The as-made zincosilicate-MFI was initially calcined at 550 °C in air overnight. Following this, the zeolite was ion exchanged in order to remove Na+ ions. The zeolite was ion exchanged with a 1.0 M NH4NO3

10.1021/ie030357m CCC: $27.50 © 2004 American Chemical Society Published on Web 09/16/2003

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solution for 1 h and then filtered, washed with distilled water, and dried overnight in air at 100 °C. An activation in air to 550 °C was used to generate the proton form of the zeolite. 2.1.2. Aluminosilicate-BEA. A commercial zeolite β sample, Na-BEA (PQ, Si/Al ) 12), was used without further postsynthesis treatments prior to platinum loading or was ion exchanged with a 1.0 M NH4Cl [or Zn(NO3)2] solution for 24 h at 80 °C and then filtered and washed with distilled water. This procedure was repeated three times. The final materials were dried in air overnight at 100 °C. 2.1.3. Zincosilicalite-BEA (CIT-6). Chemical reagents used were tetraethylammonium hydroxide (TEAOH; 35 wt % aqueous solution, Aldrich), colloidal silica (Snowtex-40, Nissan Chemical), zinc acetate dihydrate (Fisher Chemical), and LiOH monohydrate (Aldrich). CIT-6 reaction mixtures were prepared by the following method. After TEAOH and LiOH were dissolved in distilled water, Zn(OAc)2‚2H2O was added. Next, SiO2 was added and the mixture was stirred for 2 h. The reaction mixture had the following composition: 0.05:0.03:0.65:30 LiOH‚H2O/Zn(OAc)2‚2H2O/ TEAOH/SiO2/H2O. This mixture was charged into a Teflon-lined autoclave and heated statically under autogenous pressure at 150 °C for 4 days. The samples were recovered by filtration, washed with water, and dried overnight in air at 100 °C. Several postsynthesis treatments of as-made CIT-6 such as calcination, extraction of the SDA, ion exchange, and activation in nitrogen to generate the proton form were combined differently to prepare various catalysts for the dehydrogenation of propane into propylene. To remove the occluded organic molecules (SDA), two methods were employed. Samples were calcined as normally reported for zeolites that contain occluded organic molecules. Samples were heated in nitrogen to 550 °C within 10 h and then maintained at this temperature in air for 10 h. Alternatively, the samples were extracted to remove SDA from the intrazeolitic space as follows. Organic molecules were removed by contacting the as-made CIT-6 with acetic acid at 60 °C for 3 days.10 A typical extraction condition was 4.0 g of zeolite, 240 mL of glacial acetic acid, and 400 mL of water. CIT-6 was then recovered by vacuum filtration, washed with distilled water, and dried overnight in air at 100 °C. A second extraction method was to contact the zeolite with NH4NO3 or Zn(NO3)2 with approximately 1 g of zeolite/100 mL of solution.10 An as-made CIT-6 was treated with a 1.0 M aqueous NH4NO3 or Zn(NO3)2 solution at 80 °C for 10 h. The treated sample was recovered by vacuum filtration and washed with distilled water. This procedure was repeated four times. The final material was dried in air overnight at 100 °C. The zeolite was ion exchanged with a 1.0 M NH4NO3 [or Zn(NO3)2] solution for 10 h at 80 °C and then filtered, washed with distilled water, and dried overnight in air at 100 °C. Then, an activation in nitrogen to 550 °C was sometimes used to generate the proton form of the zeolite. 2.1.4. Platinum Loading. The amount of platinum to give a final 0.5 wt % was added using the following method: 1.0 g of zeolite was mixed in a round-bottom flask with 0.009 g of Pt(NH3)4Cl2‚H2O and 3.9 g of water. The mixture was stirred and dried in a rotary evaporator under vacuum. The zeolite was then dried overnight in air at 100 °C.

2.2. Characterization. X-ray powder diffraction (XRD) patterns were collected on a Scintag XDS 2000 diffractometer using Cu KR radiation. Elemental analyses were performed by Galbraith Laboratories Inc., Knoxville, TN. Thermogravimetric analyses (TGA) were carried out on a TA Instruments 951 thermogravimetric analyzer. The samples were heated in air to 800 °C and maintained 1 h at 800 °C. Solid-state NMR spectroscopy was performed on a Bruker AM 300 spectrometer equipped with a high-power amplifier for solids. Samples were spun at the magic angle (MAS) in 7 mm ZrO2 rotors at a rate of 4 kHz. 29Si NMR spectra were obtained with a 90° pulse of 6 µs and a recycle delay of 60 s. 29Si cross-polarization (CP) NMR spectra were acquired with a short contact time of 0.1 ms, a 1H 90° pulse of 5.4 µs, and a recycle delay of 5 s. All spectra were externally referenced to tetramethylsilane, 0.0 ppm. 2.3. Catalytic Activity. The dehydrogenation of propane was carried out in a flow reactor in the presence of helium (propane/helium molar ratio ) 4) under atmospheric pressure at 525, 540, and 565 °C. In each case, the weight hourly space velocity (WHSV) was equal to 2.2 h-1. Reaction products were analyzed online by gas chromatography (with a thermal conductivity detector) using a molecular sieve and a Porapak Q column. The products were identified by comparing their retention times to the retention times of reference compounds. 3. Results and Discussion 3.1. Catalysts. Table 1 lists the catalysts used here for the dehydrogenation of propane and their preparation methods. All of the samples have an XRD pattern of a crystalline β-zeolite or ZSM-5 (data not shown). 3.2. Reactivity of Platinum-Impregnated Zincosilicate-MFI. Figure 1 shows the total conversion of propane at 525, 540, and 565 °C for Pt/Zn-MFI. As expected, the catalyst is active, very selective, and stable (Table 2).8 The yields of propylene obtained at 525 °C (23.1%), 540 °C (26.8%), and 565 °C (33%) are very close to the thermodynamic values (approximately 24%, 28%, and 33%, respectively). 3.3. Reactivity of Platinum-Impregnated Aluminosilicate-BEA. The dehydrogenation of propane was carried out over Pt/H-BEA, Pt/Na-BEA, and Pt/ZnBEA (Table 3). Pt/H-BEA is the most active and the least selective catalyst. As expected, various byproducts are formed as a result of the acid sites in H-BEA. A small amount of unidentified high molecular weight products are formed. The presence of sodium and zinc decreases the amount of acid catalysis and its byproducts and thus gives catalysts with a higher selectivity. However, the presence of sodium or zinc is not sufficient to inhibit the deactivation of the catalysts [note time on stream (TOS) in Table 3]. 3.4. Reactivity of Platinum-Impregnated CIT-6. 3.4.1. As-Made CIT-6 Contacted with NH4NO3. The reaction was carried out over four different catalysts that were contacted with NH4NO3 during their preparation: Pt/CIT-6(1), Pt/CIT-6(2), Pt/CIT-6(4), and Pt/CIT6(6) (Table 1). Initially, all of the catalysts are active (>20%) and selective for propylene (>90%) (Figure 2 and Table 2). Indeed, propane is mainly transformed into propylene and hydrogen; however, small amounts of ethane (C2), methane (C1), and ethylene (C2d) are also produced. Pt/CIT-6(1) rapidly deactivates (Figure 2a),

2924 Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004 Table 1. Preparation of the Various Pt/Molecular Sieve Catalysts catalyst

step 1

Pt/Zn-MFI Pt/H-BEA Pt/Na-BEA Pt/Zn-BEA Pt/CIT-6(1) Pt/CIT-6(2) Pt/CIT-6(2)bis Pt/CIT-6(3) Pt/CIT-6(4) Pt/CIT-6(5) Pt/CIT-6(6) Pt/CIT-6(7) Pt/CIT-6(8) Pt/CIT-6(9)

calcinatione

step 2 h

NH4NO3 Pt loadingd

NH4Clg Pt loadingd Zn(NO3)2g NH4NO3a NH4NO3a NH4NO3a Zn(NO3)2a NH4NO3a calcinatione NH4NO3a Zn(NO3)2a acetic acidc acetic acidc

Pt loadingd Pt loadingd calcinatione calcinatione Pt loadingd calcinatione NH4NO3b calcinatione calcinatione Pt loadingd NH4NO3b

step 3

step 4

activationf

Pt

NH4NO3b NH4NO3b

Pt loadingd Pt loadingd

NH4NO3b activationf Pt loading Zn(NO3)2b

activationf Pt loadingd

step 5

loadingd

Pt loadingd

Pt loadingd

Pt loadingd

a Extracted with a 1.0 M aqueous solution at 80 °C for 10 h. Repeated four times. b Ion exchanged with a 1.0 M aqueous solution at 80 °C for 10 h. c Extracted with acetic acid at 60 °C for 3 days: 4.0 g of CIT-6/240 mL of acetic acid/400 mL of water. d 1.0 g of zeolite/0.009 g of Pt(NH3)4Cl2‚H2O/3.9 g of water. e Calcined in air at 550 °C for 10 h. f Activated in nitrogen at 550 °C for 10 h. g Ion exchanged with a 1.0 M aqueous solution at 80 °C for 24 h. Repeated four times. h Ion exchanged once with a 1.0 M aqueous solution for 1 h.

Table 3. Dehydrogenation of Propane on Pt/H-BEA, Pt/Na-BEA, and Pt/Zn-BEA (0.40 g of Catalyst, 540 °C, C3:He ) 4:1, WHSV ) 2.2 h-1)

catalyst Pt/H-BEA Pt/Na-BEA Pt/Zn-BEA

Figure 1. Total conversion of propane versus reaction time (min) on Pt/Zn-MFI: 0.40 g of catalyst, C3:He ) 4:1, WHSV ) 2.2 h-1. Table 2. Dehydrogenation of Propane on Pt/ Zincosilicate-MFI and on Pt/CIT-6 (0.40 g of Catalyst, 540 °C, C3:He ) 4:1, WHSV ) 2.2 h-1)

catalyst Pt/Zn-MFI Pt/CIT-6(1) Pt/CIT-6(2) Pt/CIT-6(2)bis Pt/CIT-6(3) Pt/CIT-6(4) Pt/CIT-6(5) Pt/CIT-6(6) Pt/CIT-6(7) Pt/CIT-6(8) Pt/CIT-6(9)

TOS conv (min) (%) 190 430 190 430 190 430 190 430 190 430 190 430 190 430 190 430 190 430 190 430 190 430

27.1 27.4 18.2 9.1 23.9 25.1 24.2 24.9 18.8 11.8 20.9 17.2 20.1 19.0 24.9 23.9 22.5 19.7 7.3 1.2 8.7 4.1

selectivity yield yield yield yield of C3d of C3d of C2 of C1 of C2d (%) (%) (%) (%) (%) 94.5 96.3 92.3 93.4 95.4 98.4 94.7 96.8 90.4 94.1 94.4 96.0 94.0 96.3 94.8 96.2 93.3 96.9 91.8 100 85.0 90.2

25.6 26.4 16.8 8.5 22.8 24.7 22.9 24.1 17.0 11.1 19.7 16.5 18.9 18.3 23.6 23.0 21 19.1 6.7 1.2 7.4 3.7

0.7 0.4 0.4 0.1 0.5 0.3 0.7 0.3 0.4 0.1 0.4 0.2 0.4 0.1 0.6 0.3 0.6 0.2 0 0 0.3 0

0.8 0.6 0.7 0.4 0.6 0.1 0.6 0.5 1 0.4 0.6 0.3 0.6 0.4 0.7 0.5 0.8 0.4 0.4 0 1 0.4

0 0 0.3 0.1 0 0 0 0 0.4 0.2 0.2 0.2 0.2 0.2 0 0.1 0.1 0 0.2 0 0 0

Pt/CIT-6(4) and Pt/CIT-6(6) deactivate slowly (Figure 2c,d), and Pt/CIT-6(2) is the only stable catalyst after 10 h of reaction (Figure 2b). Pt/CIT-6(2)bis [repeat preparation of Pt/CIT-6(2)] has the same catalytic behavior as Pt/CIT-6(2) (Tables 1 and 2). These results show that the catalyst preparation is critical to the stable performance, and the preparation method that

TOS conv (min) (%) 20 50 20 50 20 50

45.5 34.9 30.5 22.7 27.0 22.1

selectivity yield yield yield yield of C3d of C3d of C2 of C1 of C2d (%) (%) (%) (%) (%) 30.8 58.8 35.7 63.4 50.0 71.4

14.0 20.5 10.9 14.4 13.5 15.8

12.6 4.4 9.8 3.5 6.7 2.5

18.6 9.7 9.3 4.1 6.2 2.9

0.3 0.3 0.5 0.7 0.6 0.9

yields a stable catalyst is reproducible. Finally, the yields of propylene obtained at 525 °C (21.1%), 540 °C (24.7%), and 565 °C (31.3%) are close to the thermodynamic values and to the yields obtained over Pt/ZnMFI. 3.4.2. Other Preparation Methods of Pt/CIT-6. Pt/ CIT-6(5) was obtained by calcining an ion-exchanged (NH4NO3) CIT-6 to generate the proton form prior to loading with platinum (Table 1). Initially, the catalyst is less active than Pt/CIT-6(2) (20.1% and 23.9%, respectively; Table 2) and also deactivates slowly to reach a total conversion of 19% after 430 min of reaction (Figure 3), whereas Pt/CIT-6(2) is still stable at this time (Figure 2b). Pt/CIT-6(3) and Pt/CIT-6(7) [contacted with Zn(NO3)2 prior to platinum loading] were investigated also for the dehydrogenation of propane (Table 1). The catalytic behavior of Pt/CIT-6(3) is similar to Pt/CIT-6(1) (Figure 4a versus Figure 2a): the catalyst is quite active and selective but deactivates rapidly. Pt/CIT-6(7) is active, but the conversion decreases slowly with TOS (Figure 4b). Parts a and b of Figure 5 show the total conversion of propane at 525, 540, and 565 °C for Pt/CIT-6(8) and Pt/CIT-6(9). These samples were contacted with acetic acid at 60 °C to remove all of the zinc from CIT-6 (Table 1). Both of these catalysts show a low conversion and deactivate rapidly with TOS (Table 2). 3.5. Characterization of As-Made CIT-6 Contacted with NH4NO3. TGA experiments on the treated CIT-6 show that 42% of SDA is removed after contact with NH4NO3. This result is in agreement with the previous data of Takewaki et al.10 The elemental analyses show that the Si-to-Zn ratio is roughly the same in the as-made CIT-6 and in the catalysts [Pt/CIT-6(1), Pt/CIT-6(2), Pt/CIT-6(4), and Pt/ CIT-6(6)] before and after reaction (Table 4). Addition-

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Figure 2. Total conversion of propane versus reaction time (min) on (a) Pt/CIT-6(1), (b) Pt/CIT-6(2), (c) Pt/CIT-6(4), and (d) Pt/CIT-6(6): 0.40 g of catalyst, C3:He ) 4:1, WHSV ) 2.2 h-1.

Figure 3. Total conversion of propane versus reaction time (min) on Pt/CIT-6(5): 0.40 g of catalyst, C3:He ) 4:1, WHSV ) 2.2 h-1.

ally, the percentage of platinum in the zeolite before and after reaction is close to the expected value of 0.5% (data not shown). Thus, no volatility of zinc and platinum is observed to occur during the dehydrogenation reaction of propane. The 29Si MAS NMR spectra of materials at different stages in the preparation of Pt/CIT-6(2) (the most stable catalyst) and Pt/CIT-6(2) after reaction are shown in Figure 6. In all spectra, the spectral intensity upfield of -105 ppm is assigned to Q4 tetrahedral silicon (all four neighboring sites in the framework are occupied by other silicon atoms).9,10 There are also several peaks observed in the range between -90 and -105 ppm. The comparison of the 1H-29Si CP/MAS spectrum and the non-CP spectrum of CIT-6 contacted with NH4NO3 (Figures 7 and 6b) suggests the assignment of the peaks in the -90 to -105 ppm range to Q3 and Si(1Zn) sites. The CP spectrum shown (Figure 7) uses very short cross-relaxation periods in order to generate the 29Si

NMR signal for only the silicon atoms with close contact to a hydrogen atom. In the 1H-29Si CP/MAS spectrum (Figure 7), the peak at -98 ppm is not enhanced while the others (at -95 and -102 ppm) are enhanced. Thus, the peak at -98 ppm is assigned to Si(1Zn) sites. These data are consistent with the previous results obtained by Takewaki et al.9,10 The peak at -98 ppm is present in all of the spectra of materials at different stages of preparation of Pt/CIT-6(2) and before and after reaction with propane (Figure 6). However, this peak is smaller in the spectra of Pt/CIT-6(2) after reduction (Figure 6f) and Pt/CIT-6(2) after reaction (Figure 6g). These results show that, after all of the postsynthesis treatments to CIT-6 that give the more stable catalyst (Pt/CIT-6(2)), a portion of the zinc remains in the lattice. Curiously, a peak at -104 ppm appears in the spectra of the extracted and then calcined and extracted sample (Figure 6d) and of Pt/CIT-6(2) (Figure 6e) that disappears after reduction of the catalyst (Figure 6f). In the CP spectra of these samples (not shown), the peak at -104 ppm is not enhanced. Thus, it is not assigned to Q3 sites. This peak is also not observable in the spectrum of less stable catalysts, e.g., Pt/CIT-6(1). No assignment of this resonance can be made at this time. 3.6. Characterizations of Other Catalyst Preparations. 3.6.1. Calcined CIT-6. The calcination of the as-made CIT-6 removes 100% of the SDA. Elemental analyses (Table 4) show that the Si-to-Zn atomic ratio in Pt/CIT-6(5) is the same before and after reaction. However, previous results of Takewaki et al.10 have shown that the zinc can be removed from the framework after calcination at high temperature. Pt/CIT-6(5) has the same amount of zinc in the solid as the more stable catalyst Pt/CIT-6(2) that was contacted with NH4NO3. However, in the case of Pt/CIT-6(2), at least a part of the zinc is in the framework (from 29Si NMR results).

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Figure 4. Total conversion of propane versus reaction time (min) on (a) Pt/CIT-6(3) and (b) Pt/CIT-6(7). 0.40 g of catalyst, C3: He, 4:1, WHSV ) 2.2 h-1.

Figure 5. Total conversion of propane versus reaction time (min) on (a) Pt/CIT-6(8) and (b) Pt/CIT-6(9): 0.40 g of catalyst, C3:He ) 4:1, WHSV ) 2.2 h-1. Table 4. Si-to-Zn Atomic Ratio in the Gel, in the As-Made CIT-6, and in the Catalyst before and after Reaction

catalyst

synthesis mixture

Pt/CIT-6(1) Pt/CIT-6(2) Pt/CIT-6(3) Pt/CIT-6(4) Pt/CIT-6(5) Pt/CIT-6(6) Pt/CIT-6(7) Pt/CIT-6(8) Pt/CIT-6(9)

33.8 34.1 34.1 33.8 33.8 33.8 33.8 33.8 33.8

a

as-made

before propane reaction

after propane reaction

nda 12.3 12.3 nd nd nd 16.4 16.5 16.5

13.7 16.4 5.4 14.6 12 10.2 8.1 >3700 >3700

14.3 17.7 5.5 14.9 13.8 11.4 8.6 nd nd

nd denotes not determined.

Thus, the presence of framework zinc seems to be necessary for producing a stable catalyst such as Pt/ CIT-6(2). 3.6.2. As-Made CIT-6 Contacted with Zn(NO3)2. TGA experiments show that, after contacting the asmade CIT-6 with Zn(NO3)2, 43% of the SDA is removed. The percentage of extraction obtained for this sample is the same as that for the as-made CIT-6 contacted with NH4NO3. However, when the as-made CIT-6 is contacted with Zn(NO3)2 [Pt/CIT-6(3) and Pt/CIT-6(7)], the Si-to-Zn atomic ratio is smaller than that in the as-made CIT-6 as expected (Table 4). Moreover, the Si-to-Zn ratio is still the same as that on the used catalysts. Thus, the treatment with Zn(NO3)2 increases the amount of zinc in CIT-6. The 29Si MAS NMR spectra of materials at different steps in the preparation of Pt/CIT-6(7) are shown in Figure 8. The peak at -98 ppm, assigned to Si(1Zn) sites, is present in the spectra of all of the samples contacted with Zn(NO3)2 (Figure 8). However, this peak

Figure 6. 29Si MAS NMR spectra (a) for as-made CIT-6, (b) for as-made CIT-6 contacted with NH4NO3, (c) for as-made CIT-6 contacted with NH4NO3 and then calcined, (d) for as-made CIT-6 contacted with NH4NO3, calcined, and then ion exchanged with NH4NO3, (e) for Pt/CIT-6(2), (f) for Pt/CIT-6(2) after reduction, and (g) for Pt/CIT-6(2) after reaction.

is smaller in the spectrum of Pt/CIT-6(7) after reaction (Figure 8e). Although CIT-6 has been contacted with Zn(NO3)2, this treatment does not appear to increase the framework zinc content in the catalyst. Because the amount of zinc in Pt/CIT-6(7) is higher than that in Pt/ CIT-6(2) (Table 4), the reaction results from Pt/CIT-6(2) suggest that a higher amount of nonframework zinc does not improve the catalytic behavior. Finally, a peak

Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004 2927

Figure 7. 29Si CP/MAS NMR spectrum for as-made CIT-6 contacted with NH4NO3.

Figure 9. 29Si MAS NMR spectra (a) for as-made CIT-6 contacted with acetic acid and (b) for Pt/CIT-6(8).

Figure 8. 29Si MAS NMR spectra (a) for as-made CIT-6 contacted with Zn(NO3)2, (b) for as-made CIT-6 contacted with Zn(NO3)2 and then calcined, (c) for as-made CIT-6 contacted with Zn(NO3)2, calcined, and then ion exchanged with Zn(NO3)2, (d) for Pt/CIT6(7), and (e) for Pt/CIT-6(2) after reaction.

at -104 ppm appears on the spectra of the sample contacted with Zn(NO3)2, calcined, and then ion exchanged (Figure 8c) and of Pt/CIT-6(7) (Figure 8d) as in the spectra of the zeolite contacted with NH4NO3 (Figure 6d,f). 3.7. Characterizations of As-Made CIT-6 Contacted with Acetic Acid. When the as-made CIT-6 is contacted with acetic acid, 76% of the SDA is removed. Moreover, the zinc is also extracted from the zeolite. Indeed, before the treatment, the Si-to-Zn atomic ratio is equal to 16.5, and after the treatment with acetic acid, it is 3700 (Table 4). This result is confirmed by 29Si MAS NMR experiments: the spectra of extracted CIT-6 (Figure 9a) and of Pt/CIT-6(8) (Figure 9b) do not show the peak at -98 ppm assigned to Si(1Zn) sites. However, there is a broad peak between -100 and -105 ppm that is associated with the defects left by the zinc removed from the framework.10 Data shown in Figure 5 reveal that both samples previously contacted with acetic acid [Pt/CIT-6(8) and Pt/CIT-6(9)] are not active and not stable catalysts for the dehydrogenation of propane. These results suggest that the presence of zinc in the framework is necessary for producing a stable catalyst such as Pt/CIT-6(2). 4. Conclusions Platinum-impregnated CIT-6 is an excellent catalyst for the dehydrogenation of propane and shows activity

and selectivity similar to those of the platinumimpregnated zincosilicate catalysts possessing the ZSM-5 structure. However, the catalytic behavior of Pt/CIT-6 depends strongly on the postsynthesis treatments. The presence of framework zinc in the CIT-6 is important to the maintenance of catalyst activity. When the as-made CIT-6 is contacted with NH4NO3 to remove the SDA prior to platinum loading, an active, selective, and stable catalyst can be obtained. 29Si NMR studies on this catalyst show that a part of the zinc is no longer in the framework of CIT-6. When the as-made CIT-6 is contacted with Zn(NO3)2 to remove the SDA prior to platinum loading, additional extraframework is in the material. This additional amount of zinc does not produce a more stable catalyst. When the as-made CIT-6 is contacted with acetic acid at 60 °C, the SDA and also the zinc are extracted to give pure-silica CIT6. Platinum loading into this molecular sieve gives a catalyst that deactivates very rapidly. Thus, zinc is essential for the creation of a stable platinum-impregnated CIT-6 catalyst for propane dehydrogenation. Acknowledgment We thank Warren Smith of BP for suggesting that we investigate Pt/CIT-6 as a catalyst for dehydrogenation reactions and acknowledge the financial support of BP. It has been a pleasure for M.E.D. to work with Professor George Gavalas over the past decade, and we are delighted to provide this contribution in his honor. Literature Cited (1) Gregor, J. H.; Andersen, J. M.; Foley, T. D. Petrochemicals, Spring 1998; p 133. (2) Atkins, M. P.; Evans, G. R. Erdo¨ l, Erdgas, Kohle 1995, 111, 271. (3) Cosyns, J.; Chodorge, J.; Commereuc, D.; Torck, B. Maximize propylene production. Hydrocarbon Process. 1998, 77, 61. (4) Weyten, H.; Keizer, K.; Kinoo, A.; Luyten, J.; Leysen, R. Dehydrogenation of propane using a packed-bed catalytic membrane reactor. AIChE J. 1997, 43, 1819. (5) Meriaudeau, P.; Thangaraj, A.; Dutel, J. F.; Naccache, C. Studies on PtxSny bimetallics in NaY. 2. Further characterization and catalytic properties in the dehydrogenation and hydrogenolysis of propane. J. Catal. 1997, 167, 180.

2928 Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004 (6) Barias, O. A.; Holmen, A.; Blekkan, E. A. Propane dehydrogenation over supported Pt and Pt-Sn catalysts: Catalyst preparation, characterization and activity measurements. J. Catal. 1996, 158, 1. (7) Yarusov, I. B.; Zatolokina, E. V.; Shitova, N. V.; Belyi, A. S.; Ostrovskii, N. M. Propane dehydrogenation over Pt-Sn catalysts. Catal. Today 1992, 13, 655. (8) Barri, S. A. I.; Tahir, R. Chemical process and catalyst. EP0351067 and US5208201, assigned to British Petroleum Co. PLC.

(9) Takewaki, T.; Beck, L. W.; Davis, M. E. Synthesis of CIT-6, a zincosilicate with the (*)BEA topology. Top. Catal. 1999, 9, 35. (10) Takewaki, T.; Beck, L. W.; Davis, M. E. Zincosilicate CIT6: A precursor to a family of *BEA-type molecular sieves. J. Phys. Chem. B 1999, 103, 2674.

Received for review April 24, 2003 Revised manuscript received June 30, 2003 Accepted July 2, 2003 IE030357M