Selectivity improvement in the aromatization of C2–C5 alkanes using polyfunctional metallosilicate catalysts

Microporous and Mesoporous Materials 35–36 (2000) 89–98 www.elsevier.nl/locate/micromeso

Selectivity improvement in the aromatization of C –C alkanes 2 5 using polyfunctional metallosilicate catalysts Akihiko Matsuoka, Jin-Bae Kim, Tomoyuki Inui *,1 Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan Received 6 April 1999; received in revised form 25 June 1999; accepted for publication 1 July 1999 Dedicated to the late Werner O. Haag in appreciation of his outstanding contributions to heterogeneous catalysis and zeolite science

Abstract Improvement in selectivity in the aromatization of n-C H and n-C H was investigated on Pt-incorporated H– 4 10 5 12 Ga- or H–Zn-silicate (designated as H–Pt · Ga- or H–Pt · Zn-silicate) prepared by the addition of Pt at the stage of mixed gel formation for preparing the precursor of the crystals. The performance of these catalysts was compared with that on Pt-ion-exchanged H–Ga- or H–Zn-silicate (designated as Pt/H–Ga-silicate, Pt/H–Zn-silicate). The conversion of C –C saturated hydrocarbons on H–Pt · Ga- or H–Pt · Zn-silicate was lower than that on Pt/H–Ga- or 2 5 Pt/H–Zn-silicate, but still higher than that on Pt-free H–Ga- or H–Zn-silicate catalyst. On the Pt-incorporated catalysts, the selectivity to light aromatics and/or olefinic hydrocarbons increased, indicating that activities for cracking, alkylation of benzene ring, hydrogenation, and dehydrogenation caused by Pt could be moderated by incorporation of Pt into the crystals. On the other hand, activities of dehydrogenation and aromatization caused by Ga or Zn were exerted more explicitly. These are attributed to the difference of the Pt state between Pt-incorporated and Pt-ion-exchanged catalysts, which could be compared through measurements of CO adsorption and TEM observation. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Aromatization of C –C alkanes; Effect of Pt state; Metallosilicate; Pt-incorporated gallosilicate; Pt-incorporated 2 5 zincosilicate; Selectivity improvement

1. Introduction Large volume of methane and light paraffinic hydrocarbons are contained in natural gas and associated gas, respectively. Furthermore, a con* Corresponding author. Tel.: +81-722-448807; fax: +81-722-448085. E-mail address: [email protected] ( T. Inui) 1 Present address: Corporate Center for Gas and Chemical Research, Daido Hoxan Inc. 6-40, Chikko Shinmachi 2-Cho, Sakai, Osaka 592-8331, Japan.

siderable amount of butane and light naphtha components such as n-C H and n-C H is 5 12 6 14 derived from petroleum refining processes. For these comparatively chemically stable hydrocarbons, effective conversion to more valuable chemicals is of great importance. One of the most important targets of the conversion would be the synthesis of the major aromatic building blocks for the petrochemical industry, benzene, toluene, and xylenes, from natural gas instead of petroleum. Reflecting these strong demands, in the past two decades, a number of investigations in this

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field have been reported using pentasil-type zeolite [1–45] and metallosilicate [46–64] as the key catalyst component. In the early stage of these investigations, special attention was directed to the intrinsic property of Zn and Ga for the aromatization, especially in the combination of these metal ions with H-ZSM-5 by ion exchange. However, as was pointed out from the beginning [3], these kinds of catalysts have an essential instability. In particular, Zn was easily evaporated under the condition of light paraffin conversion of around 550–600°C, resulting in permanent deterioration of the catalytic activity. Even a Ga-ion-exchanged catalyst underwent this kind of deterioration, although the degree was much lower than that in Zn-ion-exchanged catalysts [3]. On the other hand, in the early stage, it was found that there was a prominent promoting effect on the aromatization of propane by loading a small concentration of Pt on H-ZSM-5 by an ion exchange method [4]. It was concluded that the Pt acts as the catalyst for dehydrogenation of propane before aromatization resulting in the marked enhancement of the conversion rate. The same conclusion was consistently reached by other researchers [6,13,17]. In order to overcome the instability of the Zn or Ga component in the catalyst, Ga-silicate and Zn-silicate were synthesized by replacing Al in the ZSM-5 framework with Ga or Zn at the stage of gel formation using the rapid crystallization method [48,65,66 ]. Combining a small amount of Pt with Ga- or Zn-silicate by ion exchange, these catalysts exhibited a high performance in light paraffin conversion [48,50,52]. It was also shown that Pt in Pt-ion-exchanged Ga-silicate plays important roles in preventing coke formation as the entrance for hydrogen spillover, and in accelerating coke combustion as the entrance for oxygen spillover during the period of regeneration [48,50,52,53]. The Pt ion-exchanged H–Ga- or H–Zn-silicate catalyst was then extended to apply to the aromatization of other saturated hydrocarbons ranging from methane to n-C H and a unique depen20 42 dence of the selectivity to aromatics on carbon number was found [52,53]. As a result, the selectivity to aromatics becomes a minimum in the cases

of n-C H and n-C H conversion, because the 4 10 5 12 conversion rate increases with an increase of carbon number in the range of C –C , accompa1 6 nied by the formation of smaller molecules due to hydrogenolysis by the action of Pt. A similar tendency has been reported for butane [21] and pentane [9] using Zn- and Ga-ion-exchanged ZSM-5. Hydrocarbons larger than C have a direct 6 aromatization route from C –C fragments via 6 8 1,6-cyclization and give an almost constant selectivity to aromatics (about 60 C-mol.%) irrespective of the carbon number [52]. As for the aromatization of methane by dehydrocondensation, the first report appeared in 1989 using a Pt-ion exchanged H-gallosilicate having a high Ga content such as an Si/Ga atomic ratio of 15 [67]. Selectivity to aromatics was as high as 95 C-mol.%, at a methane conversion of 4.2% at 700°C. However, the space–time yield of aromatics was only 46.3 g (as benzene) l−1 h−1, and it corresponds to ca. 1/5 of thermoequilibrium value for methane to benzene. Recent efforts have been concentrated on Mo-loaded H-ZSM-5 [68–82], and the active phase is considered to be Mo C, 2 which is predominantly located at the outer surface of the zeolite crystals [76 ]. The addition of CO 2 to the feed brings about an improvement in catalyst deactivation by coke deposit due to the reaction between CO and coke [82]. However, the conver2 sions of methane (6% at 12.5 h on stream) and space–time yield of aromatics (53 g benzene kg−1 h−1) [81] were still very low and deactivation with an increase of time on stream was significant, and there is still much room for improvement in this area. For conversion of n-hexane, n-heptane, noctane, or light naphtha to aromatics, Pt ionexchanged KL or alkali-beta zeolite has been studied extensively [83–97], and high performance has been achieved. In contrast, for C –C saturated 2 5 hydrocarbons, especially butane and pentane, no effective countermeasure has been undertaken to improve the selectivity. In this study, in order to moderate the decomposition activity, Pt was added at the stage of gel formation simultaneously with Ga or Zn. The conversion reactions of C –C saturated hydro2 5 carbons on these H–Pt · Ga- and H–Pt · Zn-silicates

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were carried out, and the catalytic performance compared with those of Pt-ion-exchanged H–Gaand H–Zn-silicates. Moreover, the difference in the Pt state between Pt-ion-exchanged and Pt-incorporated silicates was compared through measurements of CO adsorption [98] and TEM observation, and the effect of the difference on the catalytic performance is discussed.

2. Experimental 2.1. Catalyst preparation Ga- and Zn-silicate catalysts having Si/Ga atomic ratio 20 and Si/Zn atomic ratio 100, respectively, were prepared by replacing the Al ingredient in ZSM-5 using Ga (SO ) or ZnSO at the stage 2 43 4 of gel formation in the rapid crystallization method [65,66 ]. The crystals formed were washed with distilled water and dried. Then these were heated in air at 540°C for 3.5 h to burn off the organic template and to calcine. The calcined crystals were ion exchanged twice by an aqueous solution of 1 M NH NO at 80°C for 1 h. These were washed 4 3 with distilled water, followed by drying, and then calcined again in air at 540°C for 3.5 h. The Pt–Ga- and Pt–Zn-silicates were synthesized in the same way as mentioned above with addition of H PtCl simultaneously at the stage of 2 6 gel formation. In case of an Si/Pt atomic ratio of 220, Pt(NH ) Cl was used instead of H PtCl . 34 2 2 6 The composition of the Pt-incorporated silicates was set as follows: atomic ratios of Si/Ga and Si/Zn were 20 and 100, respectively, and Si/Pt was varied between 100, 220, and 600 for each kind of metallosilicate. The kinds of catalyst used were designated by capital letters as indicated in the caption of Fig. 1. The Pt ion exchange was conducted by treating the H–Ga- or H–Zn-silicate with an aqueous solution of 1.3×10−3 M Pt(NH ) Cl at 98°C for 3 h. 34 2 It was washed with distilled water and dried. Then the thermal decomposition of the platinum ammonium complex was carried out in an air stream of 50 ml min−1 by heating to 350°C at a constant heating rate of 3°C min−1 and holding at that temperature for 10 min. The hydrogen reduction

Fig. 1. Effect of Pt-loading method and Pt content on the propane conversion as a function of temperature: 6, catalyst A, Pt/H–Ga-silicate (0.5 wt.% corresponding to Si/Pt=640, Pt ion exchanged ); #, catalyst B, H–Pt · Ga-silicate (Si/Pt=100, Pt incorporated ); $, catalyst C, H–Pt · Ga-silicate (Si/Pt=220, Pt incorporated ); , catalyst D, H–Pt · Ga-silicate (Si/Pt=600, Pt incorporated); %, catalyst E, H–Ga-silicate (Pt free). Feed gas: C H 20 mol.% and N 80 mol.%; GHSV, 2000 h−1. 3 8 2

was then carried out in a stream of 10% H –90% 2 N (50 ml min−1) by heating to 400°C at a constant 2 heating rate of 3°C min−1 and holding at that temperature for 30 min. The Pt content in the catalyst was set at 0.5 wt.% (Si/Pt=640) [4]. 2.2. Apparatus and reaction method The conversion reaction of light paraffins was carried out by using a conventional flow-reaction apparatus under atmospheric pressure. A 0.50 g (0.8 ml ) portion of the catalyst was packed in a fused-silica tubular reactor of 8.5 mm inner diameter. The reaction gas composed of 20 mol.% light paraffins and 80 mol.% N was then allowed to 2 flow through the catalyst bed at a temperature in the range from 300 to 600°C and at a space velocity (SV ) of 2000 h−1. The products were analyzed by two FID-type gas chromatographs and a TCD-type one. The columns used were VZ-10 for gaseous hydrocarbons, Silicone OV-101 for gasoline-range hydrocarbons, and MS-5A for N and H . 2 2 2.3. Characterization of the catalysts Measurement of CO uptake was carried out by using a flow system connected to a thermal conduc-

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tivity detector under atmospheric pressure. Helium was used as a carrier gas at a flow rate of 45 ml min−1. In order to remove possible impurities such as H O and O in the carrier gas, the gas 2 2 was passed through a 100 ml trap packed with a 60 ml of MS-5A (60–80 mesh), which was immersed in a liquid N bath. A deoxygenator 2 packed with a supported copper catalyst at 350°C was also employed. Before CO adsorption, reduction in an H flow was carried out in situ as 2 follows. Hydrogen of 5 ml min−1 was added into the carrier gas stream, and in this manner the H 2 concentration was controlled to about 10 mol.%. The temperature was raised at a constant heating rate of 10°C min−1 from room temperature to 400°C, and maintained at that temperature for 15 min. After the system had cooled to room temperature, a CO pulse of 10 ml was injected repeatedly just in front of the catalyst bed using a microsyringe until no more irreversible adsorption could be observed. Substantial irreversible CO uptake was calculated from the chromatogram obtained. The acidity of the catalyst was measured by using the technique of temperature-programmed desorption ( TPD) of NH with a Rigaku thermo3 flex TG 8110 and a thermal analysis station TAS 1000. The Pt content in the catalysts was measured by X-ray fluorescence spectrometry with a JEOL JSX-601 spectrometer employing Rh radiation and LiF (Pt) and EDDT (Si) as diffraction crystals. The combustion of deposited coke was carried out by using the technique of temperature-programmed oxidation in an air stream with a Shimadzu micro TG-DTA 30. The transmission electron microscopy ( TEM ) micrographs were recorded with a Hitachi H800 instrument with side entry operating at 200 kV.

Fig. 2. Propane conversion and the product distribution of 550°C data in Fig. 1.

shown in Figs. 1 and 2 with Pt-ion-exchanged H– Ga-silicate (Pt/H–Ga-silicate) and Pt-free H–Gasilicate (H–Ga-silicate) for comparison. As shown in Fig. 1, every conversion of propane on the three H–Pt · Ga-silicates was lower than that on Pt/H– Ga-silicate; however, it was still greater than that on Pt-free H–Ga-silicate. For the three H–Pt · Gasilicates, the conversion of propane increased with an increase of Pt content in the catalyst; however, at higher Pt contents the difference in conversion is small. These results must reflect the state and dispersion of Pt, and the increase of conversion rate is attributed to the magnitude of activity of Pt for dehydrogenation of propane. As clearly shown in Fig. 2, the selectivity to aromatic hydrocarbons on each catalyst was almost the same, whereas the propane conversion varied. However, the distribution of aliphatic hydrocarbons less than C indicated a very significant feature. On 5 Pt/H–Ga-silicate catalyst (catalyst A), only light paraffinic hydrocarbons, mostly ethane, were formed. On the other hand, on H–Pt · Ga-silicate catalysts (catalysts B, C, and D), the selectivities of olefinic hydrocarbons such as ethylene and propylene were increased. These results indicate that activities in hydrogenation caused by Pt were moderated by incorporation of Pt in the crystals.

3. Results and discussion 3.1. Effect of Pt-loading method and Pt content on propane conversion The results of the propane conversion on three H–Pt · Ga-silicates having different Pt contents are

3.2. Performance of H–Pt · Ga-silicate catalyst in conversion of C –C paraffins 2 5 As was indicated in propane conversion, the Pt-incorporated silicate catalysts could moderate the hydrogenation activity caused by Pt. The con-

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A. Matsuoka et al. / Microporous and Mesoporous Materials 35–36 (2000) 89–98 Table 1 Comparison data for conversion of C –C paraffins on H–Ga-silicates modified with Pt by different methodsa 2 5 Feed gas

CH 2 6

Catalyst

Pt/Ga

Conversion (%) Selectivity (C-wt.%) C 1 C +paraffins 2 C +olefins 2 Aromatics

14.0 4.5 13.9 40.1 41.5

CH 3 8 Pt · Ga

n-C H 4 10

n-C H 5 12

Pt/Ga

Pt · Ga

Pt/Ga

Pt · Ga

Pt/Ga

Pt · Ga

8.7

55.3

41.6

96.2

78.8

100

97.9

3.2 5.7 39.8 51.3

3.1 31.5 7.8 57.6

7.8 16.9 19.2 56.1

3.7 44.5 5.3 46.5

8.9 23.7 14.0 53.4

2.3 42.8 5.7 49.2

7.2 24.7 12.3 55.8

a Reaction temperature: 500°C, Catalyst: Pt/Ga; Pt/H–Ga-silicate (Cat. A), Pt · Ga; H–Pt · Ga-silicate (Cat. C ).

version reaction of n-C H and n-C H , which 4 10 5 12 are easily decomposed to smaller molecules as mentioned above, was then carried out on H– Pt · Ga-silicate having an Si/Pt atomic ratio of 220 (catalyst C ). Conversion reactions of ethane and propane were also carried out for comparison. As shown in Fig. 3, although the Pt content of H– Pt · Ga-silicate was higher than that of Pt/H–Gasilicate, the conversions of C –C paraffins on H– 2 5 Pt · Ga-silicate were lower than those on Pt/H– Ga-silicate. The difference increased markedly with an increase in the carbon number of the feed paraffin. The data at 500°C shown in Fig. 3 are presented in Table 1. Compared with the data for

Fig. 3. Temperature dependence of C –C paraffin conversions 2 5 on Pt/H–Ga-silicate (catalyst A) and H–Pt · Ga-silicate (catalyst C ) catalysts: circles, C H conversion; triangles, C H conver2 6 3 8 sion; squares, n-C H conversion; pentagons, n-C H conver4 10 5 12 sion; open symbols and broken line, catalyst A, Pt/H–Gasilicate; filled symbols and solid line, catalyst C, H–Pt · Ga-silicate; half-filled symbols and dotted line, catalyst E, H–Ga-silicate. Feed gas: hydrocarbon 20 mol.% and N 80 mol.%; SV, 2 2000 h−1.

the catalyst of Pt-ion-exchanged H–Ga-silicate, a summary of the data for the Pt-incorporated H– Pt · Ga-silicate was as follows; the conversion decreased for all the paraffins fed, the selectivity to methane was always below several per cent but increased about 2–3 times except in the case of ethane conversion. However, C paraffins mark2+ edly decreased contrary to that C olefins 2+ increased except in case of ethane conversion. The selectivity to aromatics increased to a certain extent, although in case of propane conversion it did not change. Thus, the selectivity to aromatics in n-C and n-C paraffins could be increased and 4 5 approached the constant level obtained in n-C –n-C conversion [52,54]. 6 20 The reason for the difference described above must be attributed to the result that the activity of cracking or hydrogenolysis caused by Pt as well as that of hydrogenation was moderated by incorporation of Pt in the crystals. Since the cracking of the feed paraffin and the oligomers as the reaction intermediate was moderated, the cracking products decreased, leading to the higher selectivity to aromatics. The conversion reaction of n-C H without 4 10 dilution was compared at 500°C for several hours on stream with that of 20 mol.% n-C H diluted 4 10 with N . Little difference in the space–time conver2 sion of n-C H between on Pt/H–Ga-silicate (cat4 10 alyst A) and on H–Pt · Ga-silicate (catalyst C ) is shown, but the selectivity to aromatics on the latter catalyst was still higher than that on the former. Therefore, the space–time yield of aromatics on the latter catalyst was higher than that on the former.

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Fig. 4. Temperature-programmed combustion profiles for the coke deposited on catalysts A and C after n-C H conversion. 4 10 Flow rate of air, 40 ml min−1; heating rate, 5°C min−1.

After this reaction run, the amount of deposited coke was determined by measuring the weight loss during the temperature-programmed oxidation with air. The results are shown in Fig. 4. The integrated areas of the profiles around 400–600°C correspond to the amount of deposited coke. It is clear that the amount of deposited coke on the Pt-incorporated silicate was smaller than that on Pt/H–Ga-silicate, although the amount of aromatics produced on the Pt-incorporated silicate was larger than that on Pt/H–Ga-silicate. From this result, it is predicted that the rate of deactivation by coke deposit on the Pt-incorporated silicate would be slower than that on Pt/H–Ga-silicate. 3.3. Catalytic performance of the H–Pt · Zn-silicate catalyst in n-C H and n-C H conversions 4 10 5 12 Zinc is not only low cost but also Zn incorporated in MFI zeolite structure exhibits a strong attraction action of hydrogen [8]. Therefore, the H–Pt · Zn-silicate having an Si/Zn atomic ratio of 100 and an Si/Pt atomic ratio of 600 was prepared and applied to conversions of n-C H and 4 10 n-C H . 5 12 As shown in Fig. 5, although the Pt contents of both catalysts were almost the same, the conversion on the H–Pt · Zn-silicate was lower than that on Pt/H–Zn-silicate. However, it is clear that the conversion on the H–Pt · Zn-silicate was higher than that on Pt-free H–Zn-silicate. These results

Fig. 5. Temperature dependence of n-C H and n-C H con4 10 5 12 versions on Zn-containing silicates: circles, n-C H ; triangles, 4 10 n-C H ; open symbols, Pt/H–Zn-silicate; filled symbols, H– 5 12 Pt · Zn-silicate; half-filled symbols, H–Zn-silicate.

consistently indicate that the activity of Pt is moderated by incorporation of Pt in the crystals. As shown in Table 2, compared with that on Pt/H–Zn-silicate at the same reaction temperature, the conversion of n-C H on the H–Pt · Zn-silicate 4 10 was markedly lower; however, the selectivity to aromatics on both catalysts was similar. The distribution of aromatics formed exhibited a very significant feature. On Pt/H–Zn-silicate, xylenes comprised about 50% of the aromatics. In contrast, on the H– Pt · Zn-silicate, light aromatics such as benzene, toluene, and xylenes were produced in roughly equal amounts, and heavy aromatics above A were 9+ produced very little. For aliphatic hydrocarbons less than C , the Pt-incorporated silicate produced light 5 olefinic hydrocarbons such as ethylene and propylene more than Pt/H–Zn-silicate. The results of the n-C H conversion exhibited similar features to 5 12 those of the n-C H conversion. However, conver4 10 sion of n-C H was considerably higher than that 5 12 of n-C H , and selectivity to toluene from the 4 10 former was much higher than that from the later. In contrast, xylenes from n-C H were higher than 4 10 those from n-C H , indicating that the direct dimer5 12 ization route from n-C H assists the increase of 4 10 xylene formation. 3.4. Difference of the Pt state between Pt-ionexchanged H–Ga-silicate and Pt-incorporated H– Pt · Ga-silicate The conversion reaction of light paraffinic hydrocarbons occurs on both the acidic sites and

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Table 2 Comparison of catalytic performance for conversion of n-butane and n-pentane on Zn-containing silicates modified with Pt in different waysa Feed gas

n-C H 4 10

Catalyst

H–Zn

Conversion (%) Selectivity (C-wt.%) C 1 Paraffin Olefin Aromatics Aromatic distribution (C-wt.%) Benzene Toluene Xylene Other A 8 A 9+

n-C H 5 12 Pt/Zn

Pt · Zn

Pt/Zn

Pt · Zn

7.3

67.5

42.0

88.8

49.5

2.3 12.4 74.9 10.4

0.5 27.2 37.1 35.2

1.3 23.6 41.0 34.1

0.7 40.6 19.4 39.3

1.3 34.2 32.9 31.4

17.7 20.0 56.7 5.5 nil

14.7 19.1 51.3 7.5 7.4

22.2 33.6 37.5 3.4 3.3

19.3 44.3 21.9 4.0 10.5

22.0 37.4 32.8 3.9 3.9

a Reaction temperature: 500°C, Catalyst: H–Zn; H–Zn-silicate, Pt · Zn; H–Pt · Zn-silicate.

the metallic sites. In order to estimate the degree of contribution of the different kinds of active sites to the reaction, the acidities of Pt/H–Ga-silicate (catalyst A) and H–Pt · Ga-silicate (catalyst C ) were compared by means of TPD of ammonia. As shown in Fig. 6, both profiles almost coincide with each other indicating that the acidity of each catalyst was almost the same. A very small difference between them would be explained as follows. Since the platinum ion is rather large to access the pore mouth of pentasil silicate, most of the Pt atoms must be located on the outer surface of the crystals by ion exchange [4], and coalesce to ca.

Fig. 6. Ammonia TPD profiles for Pt/H–Ga-silicate (catalyst A) and H–Pt · Ga-silicate (catalyst C ). Solid line, catalyst A; dotted line, catalyst C.

3 nm Pt particles during the preparation procedures [48,53,54]. As a result, acid sites of the outer surface are selectively reduced. Any way, the number of acid sites and their strength are not changed significantly by ion exchange for a low loading of Pt. Therefore, the reason for the difference in the catalytic properties described above must be attributed to the difference in the Pt state in the catalyst. Then, in order to estimate the apparent diameter of Pt particles the content of Pt and the reduced surface of Pt particles were measured by X-ray fluorescence spectrometry and CO adsorption, respectively. The results are listed in Table 3. The contents of Pt in three H–Pt · Ga-silicates were higher than that of Pt/H–Ga-silicate. However, CO uptakes of these H–Pt · Ga-silicates were much lower than that of Pt/H–Ga-silicate. Fig. 7 shows transmission electron microscopy ( TEM ) micrographs of both catalysts observed at an original magnification of 200 000×. In the domain of crystallites of Pt/H– Ga-silicate many small particles of about 4 nm can be seen, and this size coincides with the diameter calculated from CO uptake data shown in Table 3. In contrast, no particles can be seen in the domain of crystals of H–Pt · Ga-silicate. Therefore, Pt in H–Pt · Ga-silicate would be highly dispersed inside the crystals. This difference consistently supports

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Table 3 Pt content and CO uptake for H–Ga-silicate modified with Pt by different methods and concentrations Catalyst

Pt charged (wt.%)

Pt content obs. (wt.%)

CO uptake (ml per g-cat.)

Pt/H–Ga-silicate Pt-involving Si/Pt=100 Si/Pt=220 Si/Pt=600

0.5

0.67

138

3.1 1.5 0.54

1.8 1.4 0.72

35.9 12.9 13.8

Apparent particle diameter of Pt (nm)a 4.3 45 97 47

a Calculated from CO uptake.

Fig. 7. Transmission electron micrographs of Pt/H–Ga-silicate (catalyst A) and H–Pt · Ga-silicate (catalyst C ). Original magnification, 200 000×.

the reason for the moderation of the catalytic activities caused by Pt. Any contradiction between the large apparent particle size deduced from the low value of CO uptake and the high dispersion in the crystals for H–Pt · Ga-silicate would be explained by the fact that the Pt atoms or very small clusters dispersed in the domain of crystal are anchored by surrounding oxygen atoms and strongly restrict the CO uptake.

4. Conclusion Pt-ion-exchanged H–Ga- or Zn-silicate having MFI structure exhibits a high performance for

converting saturated hydrocarbons to aromatics. The ten-oxygen member ring pore structure connected three dimensionally leads to shape selectivity to benzene and simple alkylated aromatics with a lower degree of coke formation. Ga or Zn exerts strong hydrogen-attracting action, which prevents hydrogenation of olefins, and consequently, enhances aromatization. Platinum has a role of dehydrogenation of paraffins to make olefins and the entrance for hydrogen spillover, which prevents or moderates coke formation. However, for the fairly unstable n-butane and n-pentane, the effect of Pt loaded by ion exchange is too strong and enhances hydrogenolysis, resulting in a low selectivity to aromatics. In order to balance the multicatalytic function matching to the properties of butane and propane, the too strong catalytic function of Pt was moderated by the incorporation into the crystal domain of Ga-silicate with high dispersion through the crystallization procedure, and the selectivity to aromatics could be approached to a level obtained in the n-C to 6 n-C conversion. 20

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