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Influence of structure and acidity-basicity of zeolites on platinum supported catalysts of n-C6 aromatization
applied catalysis ELSEV 1ER
A
AppliedCatalysisA: General 112 (1994) 105-115
Influence of structure and acidity-basicity of zeolites on platinum supported catalysts of n-C6 aromatization Jia-Lu Dong, Jian-Hua Zhu, Qin-Hua Xu* Chemistry Department, Nanjing University,Nanjing 210008, China (Received 25 May 1993;revised 7 February 1994;accepted 15 February 1994)
Abstract
L, Y zeolites were used as carrier to support Pt, and the catalytic properties of the supported Pt catalysts in n-hexane aromatization were investigated. The pore system of L zeolite proved to be beneficial for Pt clusters, exhibiting a high benzene selectivity in n-hexane aromatization. The secondary pores of zeolite L had a negative influence on the aromatization over Pt/BaKL. With these secondary pores blocked, the 0.4% Pt/BaKL-I sample could keep up with the excellent aromatization properties of 0.6% Pt/BaKL zeolite. Pt dispersed on KL basic zeolite exhibited dehydrogenation activity in isopropanol decomposition. Pt supported on NH4L acidic zeolite enhanced the conversion of isopropanol to form water and propene. IR results of linear CO adsorbed on Pt/L zeolites suggested the Pt on NH4L acidic zeolite was "electron deficient", and Pt on KL basic zeolite exhibited some electron excess, which results from the interaction of Pt and zeolites with different acidic-basic properties. Keywords: Hexane aromatization;Isopropanoldecomposition;Platinum/zeolite;zeolites
1. Introduction
Since the P t / B a K L was reported to exhibit excellent high activity and selectivity in hexane aromatization [ 1 ], several papers have been published on this subject. Derouane and Vanderveken [ 2] have proposed that the high selectivity of Pt/BaKL catalyst in aromatization should be attributed to the special pore structure of zeolite L, which confined n-hexane molecules within its pore in a "pseudocycle" manner [2], but later they found that the n-hexane aromatization selectivity on Pt/ Mg(AI) O, without unique pore structure as molecular sieves, was similar to that *Correspondingauthor. 0926-860X/94/$07.00 © 1994ElsevierScienceB.V. All rights reserved SSDI O926-860 X 0926-860X(94)00034-O
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Jia-Lu Dong et al. /Applied Catalysis A: General 112 (1994) 105-115
of Pt/KL [3]. Tauster et al. [4] suggested that the high selectivity of Pt/KL in aromatization came from the ability of the zeolite to collimate n-hexane molecules and produce terminal adsorption, and they implied that terminal cracking and high aromatization were linked. Mielazarski et al. [5] based on their investigation, however, claimed that the confinement model suggested by Derouane et al. was an unsatisfactory explaination for the excellent performence of Pt/KL, and they found that a correlation of terminal cracking index with benzene selectivity existed for both microporous and nonmicroporous materials which was in contradiction to the molecular die hypothesis of Tauster et al. [4]. They proposed that the uniqueness of Pt/KL for n-hexane aromatization was due to the ability of the L zeolite framework to stabilize extremely small Pt clusters in a completely nonacidic environment [5]. The function of molecular sieves in Pt supported aromatization catalysts, however, is still not clear and sometimes contradictory in different works. In this paper, L and Y zeolites with different structure were used as supports, and the catalytic properties of these supported Pt catalysts in n-hexane aromatization were investigated. To study the influence of acidic and basic properties of zeolite on the size distribution and state of the Pt clusters, isopropanol decomposition was employed as a probe reaction.
2. Experimental 2.1. Materials and reagents KL zeolite, with the chemical composition 1.1 K20:AI203:5.48 SiOz:6 H20, was synthesized according a patented method [6]. Then it was ion exchanged to BaKL according to the procedure given previously [7]. NHaL was prepared by ion exchange of KL zeolite with NH4NO3 solution followed by drying at 393 K. Pt was loaded onto the support materials using' 'drying impregnation" with Pt (NH3) 2CL2 or KzPtC16. The amount of Pt loaded was 0.6 wt.-% on all samples. Prior to being used in the catalytic reaction or H2-TPD measurement, these Pt loaded samples were' 'in situ" heated and kept at 523 K for 2 h in air to decompose the Pt compound, then they were "in situ" reduced in hydrogen from room temperature (r.t.) to 753 K at a rate of 8 K/min. The H2-TPD experiments were carried out in a fixed bed, flow-type apparatus with a TCD, and the results showed that the Pt dispersion ( H / Pt) of Pt/BaKL, Pt/KL and Pt/NH4L were all above 1. NaY zeolite with SIO2/A1103 of 5.02 was provided by Wenzhou Factory of Catalysts. KY was prepared by ion exchange with KNO3 solution before it was loaded with Pt or ion exchanged to BaKY. SiOz with more than 10 nm of pore size was a product of Nanjing Inorganic Factory. n-Hexane, isopropanol and other agents used in the experiments were AR purities.
Jia-Lu Dong et al. / Applied Catalysis A: General 112 (1994) 105-115
107
The purities of hydrogen and helium were better than 99.9%, and they were purified with deoxidizer and 5A molecular sieves before being used.
2.2. Appartus and procedures n-Hexane aromatization was performed on a conventional continuous flow microreactor as has been reported previously [7] at 768 K and standard atomosphere, with a W H S V of 2.20 h - ~. Hydrogen was the reaction carrier gas with a flow rate of 0.48 mo1/gcata j h. Liquid n-hexane was pumped into a quartz reactor containing the catalyst ( - 2 0 / + 40 mesh). The products were detected with TCD of "on line" SP3700 GC after being seperated on a DNP column ( Q 3 × 4 0 0 0 mm). Conversion was calculated as the mole percent of n-hexane reacted and selectivity was calculated by dividing the yield of a product by the conversion of n-hexane. In the decomposition of isopropanol with a WHSV of 3.30 h - 1, products were seperated in parapak T column (•3 × 4000 mm). The analysis data were normalized using a HP3390A integrator. Ft-IR measurments were carried on a Nicolet 510P spectrometer, and an in situ stainless steel 1R cell in the "flow system" was used. The sample disc of 10 mg/ cm 2 was activated or reduced "in situ" before it adsorbed CO at r.t. or pyridine at 423 K. All IR spectra were recorded at the given temperature, and the spectral resolution was 4 cm - 1. The adsorption properties of samples were measured using a conventional gravimatic adsorption method.
3. Results and discussion
3.1. Effect of zeolites channel structure Both P t / B a K L and Pt/SiO2 were monofunctional catalysts, but their catalytic properties were quite different in n-hexane aromatization (Table 1). Compared Table 1 Catalytic properties of n-hexane aromatization over Pt supported catalysts Sample
PtKL a
Pt/SiOz a
Pt/KY b
Pt/KL b
Conv. (mol-%) Se|ec. for Benzene (mol-%) Isomate (mol-%) C5/C,,
31.6 70.9 4.7 10.2
21.2 59.1 31.3 3.3
48.6 63.2 17.2
42.1 73.7 0.5
aLoaded with K2PtCI6. bLoaded with Pt ( NH 3) 2C12.
108
Jia-Lu Dong et al./Applied Catalysis A: General 112 (1994) 105-115 b ,""I 30 0 o
r -.I 1
× 20
a
i i-
lo
e..]
i
i
1.0
' 3.0
~'L¢'~-'i
'
]
,1 '
.I
5.0 7.'o 9.'o 11.o 13.0 Pt particte size distribution (nm)
Fig. 1. Platinum particle size distribution (nm). (a) Pt/SiO2, (b) Pt/KL.
with Pt/KL, Pt/SiO2 showed lower aromatization activity and selectivity for benzene, as well as a lower C5/C4 ratio; but a higher isomedzation component in the product distribution. According to Derouane et al. [2], in the channels of Pt/KL, n-hexane molecules were 1,6 terminal adsorbed on the active site of the Pt cluster, and dehydrocyclizated to benzene. Meanwhile, part of the adsorbed n-hexane could form C5 through a terminal cracking process, and increased the C5/C4ratio. Macroporous Pt/SiO2 could not provide such a geometric effect, so that the n-hexane molecule mainly 1,5 nonterminal adsorbed and, as Tauster et al. [4] suggested, C6 isomers consequently formed while more C4 compounds were produced in cracking. Another factor causing a catalytic difference between Pt/KL and Pt/SiO2 was the particle sizes of Pt. Those of Pt/SiO2 were distributed in the range 2-10 nm and were larger than those of Pt/K1 which were in the range 1-3 nm (Fig. 1), although two samples were loaded Pt in the same procedures. The isomerization activity of Pt/KY was found to be much more than that of Pt/KL, as seen in Table 1, and the main isomer was MCP. Gallezot et al. reported [ 8 ] that on the reduced sample the main active sites of dispersed Pt were located in the supercage of the Y zeolite. The supercage of the Y zeolite had a larger free space than that in L zeolite, so that the reaction intermediates and products would encounter less hindrance in Pt/KY than in Pt/KL; therefore, hexane molecules were easier to form MCP on the Pt cluster by 1,5 non-terminal adsorption. Two compounds, K2PtC15 and Pt(NH3)2CL2, were loaded on BaKY and BaKL zeolites; and the formed catalysts exhibited different properties in n-hexane aromatization, as shown in Table 2. The sample made with Pt(NH3)2C12 had a higher activity and selectivity than that loaded with K2PtC16. The reason, in our opinion, was that the configuration of K2PtCI6 consisted of cubical octahedra, but Pt(NH3)2CI 2 had a plane quadrilateral configuration which could be distorted easily; so that Pt(NH3)2C12 could enter the channel of the zeolite, but the K2PtCI6 could only partly enter it; which was proved with 129Xe-NMR experiments by Yang et al. [9]. The different location of the two Pt compounds on zeolite affected the disperion of Pt. For example, no Pt particle larger than 4 nm was observed on the TEM pictures of B aKL loaded with Pt (NI-I3 )2C12, and most of the Pt particles had a size in the range 2-3 nm, but on the BaKL loaded with KzPtC16, the ratio of Pt
Jia-Lu Dong et al. / Applied Catalysis A." General 112 (1994) 105-115
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Table 2 Catalytic properties of BaKL and BaKY zeolites loaded with K2PtCI6 or Pt(NH3)2CI 2 Soaking solution
BaKL
K2PtC% Pt(NH 3)2C12
BaKY
Conv.(%)
Selec.(%) a
Conv.(%)
Selec.(%) a
23.5 99.4
65.8 93.1
I 1.4 52.4
49.5 63.1
~The selectivity for benzene in the n-hexane aromatization.
particles with a size larger than 4 nm was found to be 36.5%. As a result, different catalytic properties existed on these Pt supported zeolites with these two kinds of Pt compounds.
3.2. The function of secondary pores in zeolites Sample KL-I was not washed neutral but kept at strong basic conditions ( p H = 13) in the synthesis process; then it was ion exchanged with Ba 2+ and loaded with Pt(NH3)2C12 as described before. In contradiction to that reported by Mielazarski et al. [ 5 ], the Pt/BaKL-I zeolites kept their high benzene selectivities (more than 94%) at different n-hexane conversion in aromatization of n-hexane (Table 3). Besides, 0.4% Pt/BaKL-I catalyst was found to retain as excellent aromatization properties as those of 0.6% Pt/BaKL (Fig. 2). Different from BaKL zeolite, a new peak with a 20 value of 23.5 appeared on the XRD pattern of BaKL-I, and according to ASTM [ 10] it should be attributed to BaSiO3. Chemical analysis showed that the ratio of BaO/A1203 was higher on BaKL-I than on BaKL (Table 4) which resulted from the excess BaSiO3, and Ba(OH)2 etc. existed on the BaKL-I sample. The adsorption isotherm of benzene on the BaKL-I sample was found to be a typical Langmuir isotherm, without capililary condensation at higher P/Po as found in that of the BaKL sample (Fig. 3); which revealed that impurities such as BaSiO3 had blocked the secondary pores Table 3 Conversion and product distribution from reaction of n-hexane on Pt/BaKL-I samples Reaction conditions: 763 K, WHSV = 2.20 h-% 5 h Amount of Pt loaded Conversion of n-hexane Select. to Benzene (wt.%) (mol-%) (%)
Select. to C~-C5 (%)
Select. to C6 isomers (%)
0.2 0.3 0.4 0.6 0.8 1.1
2.3 2.7 2.2 2.6 1.7 2.1
3.5 1.3 0.1 0.4 0.1 0.2
76.1 83.2 94.2 91.0 96.1 95.2
94.2 96.0 97.7 97.0 98.2 97.7
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Jia-Lu Dong et al. / Applied Catalysis A: General 112 (1994) 105-115
loo
// E 8o-
8
/
>~ 6 0
~ 4o I
c
it
/: /
20-
i
l
02
l
I
I
0.4 06 0.8 1.0 Amount of Pt loaded ( w t . - * / , )
Fig. 2. Conversion of n-hexane on (A) Pt/BaKL-I and (B) Pt/BaKL zeolites in aromatization (768 K) vs. amount of platimum loaded. Table 4 Chemiscal analysis data of L zeolites Sample
SIO2/A1203
K20/AIzO3
BaO/AI203
BaKL BaKL-I
5.40 5.81
0.80 0.92
0.30 0.53
22"
i
.is
L ~ :#5"~.o.'" It
o
.o~
t~.-
~
. . . .
-
6 E <
i
i
i
0.4
i
0.8 P I Po
Fig. 3. Adsorption isotherms of benzene on L zeolites. ( X ) KL, (o) KI-I, (o) Linde-KL, ( • ) BaKL, (z~) BaKL-I.
and part of the channels of BaKL-I, which made the capillary condensation disappear and lowered the adsorption capacity. This blocking was found to decrease not only the Pt consumed and the adsorption capacity, but also some side-reaction occurred in the secondary pores. This conclusion was confirmed by comparing Pt/ KL and Pt/KL-I catalysts. It was certain that the strong basic condition of the
Jia-Lu Dong et al. / Applied Catalysis A: General 112 (1994) 105-115
111
BaKL-I and KL-I samples formed had some influences on their dispersion and catalytic activity of the supported Pt, and further investigations are needed.
3.3. Influence of acidic-basic properties of zeolites NH3-TPD and CO2-TPD were used to determine the acidic-basic properties of NHaL and KL (Table 5). Two peaks were found in NH3-TPD of NHaL, as compared to only one low-temperature peak in that of KL zeolite, showing that the acidic strength of NHaL was greater. In the meantime, the total acidic amount of NH4L was 4.5 times larger than that of KL zeolite. NHnL zeolite exhibited strong BrCnsted acidity, as characterized by the 1540 c m - 1 band in the FT-IR spectrum of adsorbed pyridine. After being loaded with 0.6% of Pt on NHaL, the area of the 1540 c m - 1 band in the FT-IR spectrum was reduced (Fig. 4), which meant that loading of Pt lowered the BrCnsted acidity of NH4L zeolite. Isopropanol decomposition, which could be dehydrated on the acid site or dehydrogenated over the basic site [ 11 ], was employed as a probe reaction. Hydrogen was chosen as the carrier gas both in this reaction and in the aromatization, in order to correlate the catalytic behaviours of supported Pt samples in these two reactions. Isopropanol decomposition on NHaL zeolite only produced water and propene. At 498 K, almost all of the reactant had been converted. KL showed some basicity in the probe reaction and aceton formed on it. A1498 K, the isopropanol conversion and acetone selectivity on KL zeolite were 11.9% and 41.3%, respectively. The Pt loaded suppressed dehydration, but accelerated dehydrogenation of isopropanol over KL zeolite (the conversion and acetone selectivity on Pt/NH4L were 49.2% and 94.2%), which indicated the dehydrogenation function of the Pt cluster. In this reaction, Pt/NH4L, which showed a strong acidity, enhanced the conversion of isopropanol to form water and propene. It is known that on acidic centers of zeolites, toluene alkylation with methanol produced xylene, and on basic sites ethylbenzene formed [11,12]. In toluene alkylation with methanol at 693 K, the main product on NH4L zeolite was xylene, as expected, but the yield of xylene was only 1.3%. Xylene was found to be the main product over Pt/NH4L zeolite, in such alkylation, and the yield of xylene was 5.7%. It proved further that Pt loading considerably enhanced the acidic catalysis of NHaL zeolite. Table 5 TPD results of KL and NH~L zeolites CO2-TPD
NH3-TPD Samples
KL
NI-14L
KL
NH4L
Tm(K) Amount ( m m o l / g )
392 0.63
394, 656 1.12, 1.72
426 14.4.10 -2
426 5.31.10 -2
112
Jia-Lu Dong et al. / Applied Catalysis A: General 112 (1994) 105-115
The Pt/KL was also sensitive for thiophene poisoning in isopropanol decomposition: only 0.02 mmol/g of thiophene decreased acetone yield by 25% at 593 K. However, poisoning with 0.17 mmol/g thiophene only decreased the water yield by one fifth in isopropanol decomposition over Pt/NH4L at 443 K. The main active centres of Pt/NH4L zeolite in this probe reaction were found to be the acid sites, and just as those of NHaL, they were easily poisoned by pyridine (Fig. 5). Owing to the strong acidity, the NH4L zeolite deactivated quickly in isopropanol decompositon at 443 K, even in hydrogen atmosphere. After 5 h on steam, the
A
1560
i
i
i 1500 cm_iI 14 40
Fig. 4. FT-IR spectra of adsorbed pyridine on L zeolites before and after loading of platinum. (A) NH4L, purged at 423 K, (B) Pt/NFL,L, purged at 423 K.
. 0.8 ~ ._> i:i ._u
~'
o.4
o
0
~
0.026
0.052
Amountof pyrldine(rnmol/g)
Fig. 5. Influence of pyridine poison on the dehydration of isopropanol on Pt/NH4L zeolite.
Jia-Lu Dong et aL /Applied Catalysis A: General 112 (1994) 105-115
100 [
._~
~
~
70
/
~
I 13
~
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--..,
60-
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~
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1
2
~
i
i
t
3 4 5 6 Reaction t~me (h) Fig. 6. Reactivity of NH4L zeolites before and after loading of platinum in the decomposition of isopropanol (443 K) vs. reaction time. (A) Pt/NH4L zeolite in hydrogen, (B) Pt/NHnL zeolite in helium, (C) NH4L in zeolite in hydrogen.
conversion of isopropanol on NH4L decreased from 100% to 66.2%, and 2.7% coke formed on it. Oppositely, Pt/NH4L zeolite always kept its high activity under the same reaction conditions. The conversion of isopropanol was still 100% after use in steam for 6 h, and only 1.6% of coke formed. When the reaction carrier gas was changed from hydrogen to helium, the isopropanol conversion on Pt/NH4L decreased clearly from 100% to 58.1% after 5 h, as seen in Fig. 6, and 4.3% of coke formed on the sample. Similar to that observed by Chow et al. [ 13], these results showed the generally accepted scheme that Pt held the acid sites clean, i.e. coke precursors on the acid sites were removed by interacting with hydrogen. In the reaction atmosphere of helium, hydrogen no longer existed in the gas phase and the Pt cluster on NH4L could not catalyze dehydrogenation of isopropanol, so that no hydrogen formed for the Pt cluster to help the acid sites of the zeolite to remove the coke. The catalytic properties of Pt dispersed on acidic NH4L were totally different from that on basic KL zeolite. In the aromatization of the n-hexane at 768 K, Pt/ KL exhibited an exceptional selectivity of benzene (93.0%), similar to that of Pt/ BaKL (92.7%), and was very sensitive to thiophene poison. It was thus shown that both P t / K L and Pt/BaKL zeolites were monofunctional catalysts for aromatization. Pt/NH4L zeolite was proven to be a bifunctional catalyst, since in aromatization of hexane, more isomers formed on it htan aromatic products. Besides, poison with 0.12 mmol/g of thiophene decreased the n-hexane conversion over Pt/BaKL zeolite from 93.1% to 8.5%, and consequently no benzene formed at all; the same amount of thiophene, however, could not lower the benzene selectivity on Pt/NH4L zeolite in the same reaction conditions. The main CO band observed on Pt/NH4L zeolite was seen (Fig. 7) after adsorption in 2069 c m - i for linear CO, but in 2034 c m - ] with a shoulder band of 2018
114
Jia-Lu Dang et al. /Applied Catalysis A: General 112 (1994) 105-115
2140
21100
i
2060
I
2020
i
1980 cm-1
1940
Fig. 7. FT-IR spectra of carbon monoxide adsorbed on (A) Pt/NI-I4L and (B) Pt/KL zeolites.
c m - ~ on P t / K L zeolite. The difference in the electronic state of the Pt cluster was found to be the main factor causing the change of Vco on Pt supported L zeolites. It was reported by Primet et al. that in Pt/A1203 [ 14] or Pt/faujasite [ 15], a Vco vibration free of C O - C O coupling could be reached closed to 2052 c m - '. For Pt/ NH4L zeolite, the Uco value was higher than in this reference, which indicated a decrease in back donation from Pt to CO due to the Pt-zeolite interaction, since the Pt cluster interacted with the electron-acceptor acidic sites and showed a kind of "electron deficiency". In P t / K L zeolite, the CO wavenumber was lower than the reference at 2052 c m - ], which could be explained as an increase in back-donation from Pt to CO, resulting from the Pt interacting with electron donor framework atoms of the KL zeolite, such as the negatively charged lattice oxygen. Those basic sites would play the same role as electon donor [ 16] and lower Vco of CO adsorbed on Pt/KL. It is known that the anti-sulfur properties of supported Pt catalysts was proportional to the "electron deficiency" of the Pt particles [17], and Pt/NH4L zeolite exhibited a higher resistance to thiophene poison than Pt/KL in both isopropanol decomposition and aromatization; so it was proven that the Pt active centres on Pt/NH4L zeolite unlike those on P t / K L with electron excess, showed obvious "electron deficiency". The different acidic-basic properties of the zeolite changed the electronic state of the supported Pt and caused the different catalytic properties of them.
4. Concluding remarks Some conclusions emerged from this work: the pore structure of the zeolite not only affected the sizes and location of the Pt particles, but also provided hindrance for the intermediates or products to form selectively in the reactions. The pore system like zeolite L, in our opinion, was beneficial for highly dispersed Pt clusters
Jia-Lu Dong et al. / Applied Catalysis A: General 112 (1994) 105-115
115
to exhibit an excellent catalytic activity in aromatization. The acidic-basic properties of zeolites had strong effects on the electron state and location as well as the catalytic properties of Pt active centres.
Acknowledgements Supports of this work by the National Natural Science Foundation of China, and the Chinese Education Committee were greatfully acknowledged.
References [ 1 ] J.R. Hughes, W.C. Buss, P.W. Tamm and R.L. Jacobson, Stud. Surf. Sci. Catal., 28 (1986) 725. [21 E.G. Derouane and D. Vanderveken, Appl. Catal., 45 (1988) L15. [ 31 R.T. Davis and E.G. Derouane, Nature (London), 349 ( 1991 ) 313. [4] S.T. Tauster and J.J. Steger, J. Catal., 125 (1990) 387. [51 E. Mielczarski, S.B. Hong, R.J. Davis and M.E. Davis, J. Catal., 134 (1992) 359. [6] J.L. Dong and Q.H. Xu, CN 1055344A ( 1991 ). [7] J.L. Dong, C.T. Jin and Q.H. Xu, RanLiao HuaXue XueBao, 20 (1992) 244. [8] P. Gallezot, A. Alarron-Diaz, J-A. Dalmon, A.J. Renouprez and B. Imelik, J. Catal., 39 (1975) 334. [9] O.B. Yang, S.I. Woo and R.T. Ryoo, J. Catal., 123 (1990) 375. [ 10 ] 7th set of the X-ray Powder Data File, American Society for Testing Materials ( 1957 ). [ 11 ] P.E. Hathaway and M.E. Davis, J. Catal., 116 (1989) 263. [ 12] P.E. Hathaway and M.E. Davis, J. Catal., 119 (1989) 497. [ 13] M. Chow, S.H. Park and W.M.H. Sachtler, Appl. Catal., 19 (1985) 349. [ 141 M. Primet, J. Catal., 88 (1984) 273. [ 151 M. Primet, L.C. de Menorval, J. Fraissard and T. lto, J. Chem. Soc. Faraday. Trans. 1, 81 (1985) 2867. [ 161 A. de Mallmann and D. Barthomeuf, H.G. Karge and J. Weitkamp, (Editors), Stud. Surf. Sci. Catal., Vol. 46, Elsevier, Amsterdam, 1989, pp. 429-438. [ 17] R.A. Dalla Betta and M. Boudard, in J.W. Hightower (Editor), Proc. 5th International Congress on Catalysis, 1972, Vol. 2, North Holland, Amsterdam, 1972, pp. 1329-1341.
A
AppliedCatalysisA: General 112 (1994) 105-115
Influence of structure and acidity-basicity of zeolites on platinum supported catalysts of n-C6 aromatization Jia-Lu Dong, Jian-Hua Zhu, Qin-Hua Xu* Chemistry Department, Nanjing University,Nanjing 210008, China (Received 25 May 1993;revised 7 February 1994;accepted 15 February 1994)
Abstract
L, Y zeolites were used as carrier to support Pt, and the catalytic properties of the supported Pt catalysts in n-hexane aromatization were investigated. The pore system of L zeolite proved to be beneficial for Pt clusters, exhibiting a high benzene selectivity in n-hexane aromatization. The secondary pores of zeolite L had a negative influence on the aromatization over Pt/BaKL. With these secondary pores blocked, the 0.4% Pt/BaKL-I sample could keep up with the excellent aromatization properties of 0.6% Pt/BaKL zeolite. Pt dispersed on KL basic zeolite exhibited dehydrogenation activity in isopropanol decomposition. Pt supported on NH4L acidic zeolite enhanced the conversion of isopropanol to form water and propene. IR results of linear CO adsorbed on Pt/L zeolites suggested the Pt on NH4L acidic zeolite was "electron deficient", and Pt on KL basic zeolite exhibited some electron excess, which results from the interaction of Pt and zeolites with different acidic-basic properties. Keywords: Hexane aromatization;Isopropanoldecomposition;Platinum/zeolite;zeolites
1. Introduction
Since the P t / B a K L was reported to exhibit excellent high activity and selectivity in hexane aromatization [ 1 ], several papers have been published on this subject. Derouane and Vanderveken [ 2] have proposed that the high selectivity of Pt/BaKL catalyst in aromatization should be attributed to the special pore structure of zeolite L, which confined n-hexane molecules within its pore in a "pseudocycle" manner [2], but later they found that the n-hexane aromatization selectivity on Pt/ Mg(AI) O, without unique pore structure as molecular sieves, was similar to that *Correspondingauthor. 0926-860X/94/$07.00 © 1994ElsevierScienceB.V. All rights reserved SSDI O926-860 X 0926-860X(94)00034-O
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Jia-Lu Dong et al. /Applied Catalysis A: General 112 (1994) 105-115
of Pt/KL [3]. Tauster et al. [4] suggested that the high selectivity of Pt/KL in aromatization came from the ability of the zeolite to collimate n-hexane molecules and produce terminal adsorption, and they implied that terminal cracking and high aromatization were linked. Mielazarski et al. [5] based on their investigation, however, claimed that the confinement model suggested by Derouane et al. was an unsatisfactory explaination for the excellent performence of Pt/KL, and they found that a correlation of terminal cracking index with benzene selectivity existed for both microporous and nonmicroporous materials which was in contradiction to the molecular die hypothesis of Tauster et al. [4]. They proposed that the uniqueness of Pt/KL for n-hexane aromatization was due to the ability of the L zeolite framework to stabilize extremely small Pt clusters in a completely nonacidic environment [5]. The function of molecular sieves in Pt supported aromatization catalysts, however, is still not clear and sometimes contradictory in different works. In this paper, L and Y zeolites with different structure were used as supports, and the catalytic properties of these supported Pt catalysts in n-hexane aromatization were investigated. To study the influence of acidic and basic properties of zeolite on the size distribution and state of the Pt clusters, isopropanol decomposition was employed as a probe reaction.
2. Experimental 2.1. Materials and reagents KL zeolite, with the chemical composition 1.1 K20:AI203:5.48 SiOz:6 H20, was synthesized according a patented method [6]. Then it was ion exchanged to BaKL according to the procedure given previously [7]. NHaL was prepared by ion exchange of KL zeolite with NH4NO3 solution followed by drying at 393 K. Pt was loaded onto the support materials using' 'drying impregnation" with Pt (NH3) 2CL2 or KzPtC16. The amount of Pt loaded was 0.6 wt.-% on all samples. Prior to being used in the catalytic reaction or H2-TPD measurement, these Pt loaded samples were' 'in situ" heated and kept at 523 K for 2 h in air to decompose the Pt compound, then they were "in situ" reduced in hydrogen from room temperature (r.t.) to 753 K at a rate of 8 K/min. The H2-TPD experiments were carried out in a fixed bed, flow-type apparatus with a TCD, and the results showed that the Pt dispersion ( H / Pt) of Pt/BaKL, Pt/KL and Pt/NH4L were all above 1. NaY zeolite with SIO2/A1103 of 5.02 was provided by Wenzhou Factory of Catalysts. KY was prepared by ion exchange with KNO3 solution before it was loaded with Pt or ion exchanged to BaKY. SiOz with more than 10 nm of pore size was a product of Nanjing Inorganic Factory. n-Hexane, isopropanol and other agents used in the experiments were AR purities.
Jia-Lu Dong et al. / Applied Catalysis A: General 112 (1994) 105-115
107
The purities of hydrogen and helium were better than 99.9%, and they were purified with deoxidizer and 5A molecular sieves before being used.
2.2. Appartus and procedures n-Hexane aromatization was performed on a conventional continuous flow microreactor as has been reported previously [7] at 768 K and standard atomosphere, with a W H S V of 2.20 h - ~. Hydrogen was the reaction carrier gas with a flow rate of 0.48 mo1/gcata j h. Liquid n-hexane was pumped into a quartz reactor containing the catalyst ( - 2 0 / + 40 mesh). The products were detected with TCD of "on line" SP3700 GC after being seperated on a DNP column ( Q 3 × 4 0 0 0 mm). Conversion was calculated as the mole percent of n-hexane reacted and selectivity was calculated by dividing the yield of a product by the conversion of n-hexane. In the decomposition of isopropanol with a WHSV of 3.30 h - 1, products were seperated in parapak T column (•3 × 4000 mm). The analysis data were normalized using a HP3390A integrator. Ft-IR measurments were carried on a Nicolet 510P spectrometer, and an in situ stainless steel 1R cell in the "flow system" was used. The sample disc of 10 mg/ cm 2 was activated or reduced "in situ" before it adsorbed CO at r.t. or pyridine at 423 K. All IR spectra were recorded at the given temperature, and the spectral resolution was 4 cm - 1. The adsorption properties of samples were measured using a conventional gravimatic adsorption method.
3. Results and discussion
3.1. Effect of zeolites channel structure Both P t / B a K L and Pt/SiO2 were monofunctional catalysts, but their catalytic properties were quite different in n-hexane aromatization (Table 1). Compared Table 1 Catalytic properties of n-hexane aromatization over Pt supported catalysts Sample
PtKL a
Pt/SiOz a
Pt/KY b
Pt/KL b
Conv. (mol-%) Se|ec. for Benzene (mol-%) Isomate (mol-%) C5/C,,
31.6 70.9 4.7 10.2
21.2 59.1 31.3 3.3
48.6 63.2 17.2
42.1 73.7 0.5
aLoaded with K2PtCI6. bLoaded with Pt ( NH 3) 2C12.
108
Jia-Lu Dong et al./Applied Catalysis A: General 112 (1994) 105-115 b ,""I 30 0 o
r -.I 1
× 20
a
i i-
lo
e..]
i
i
1.0
' 3.0
~'L¢'~-'i
'
]
,1 '
.I
5.0 7.'o 9.'o 11.o 13.0 Pt particte size distribution (nm)
Fig. 1. Platinum particle size distribution (nm). (a) Pt/SiO2, (b) Pt/KL.
with Pt/KL, Pt/SiO2 showed lower aromatization activity and selectivity for benzene, as well as a lower C5/C4 ratio; but a higher isomedzation component in the product distribution. According to Derouane et al. [2], in the channels of Pt/KL, n-hexane molecules were 1,6 terminal adsorbed on the active site of the Pt cluster, and dehydrocyclizated to benzene. Meanwhile, part of the adsorbed n-hexane could form C5 through a terminal cracking process, and increased the C5/C4ratio. Macroporous Pt/SiO2 could not provide such a geometric effect, so that the n-hexane molecule mainly 1,5 nonterminal adsorbed and, as Tauster et al. [4] suggested, C6 isomers consequently formed while more C4 compounds were produced in cracking. Another factor causing a catalytic difference between Pt/KL and Pt/SiO2 was the particle sizes of Pt. Those of Pt/SiO2 were distributed in the range 2-10 nm and were larger than those of Pt/K1 which were in the range 1-3 nm (Fig. 1), although two samples were loaded Pt in the same procedures. The isomerization activity of Pt/KY was found to be much more than that of Pt/KL, as seen in Table 1, and the main isomer was MCP. Gallezot et al. reported [ 8 ] that on the reduced sample the main active sites of dispersed Pt were located in the supercage of the Y zeolite. The supercage of the Y zeolite had a larger free space than that in L zeolite, so that the reaction intermediates and products would encounter less hindrance in Pt/KY than in Pt/KL; therefore, hexane molecules were easier to form MCP on the Pt cluster by 1,5 non-terminal adsorption. Two compounds, K2PtC15 and Pt(NH3)2CL2, were loaded on BaKY and BaKL zeolites; and the formed catalysts exhibited different properties in n-hexane aromatization, as shown in Table 2. The sample made with Pt(NH3)2C12 had a higher activity and selectivity than that loaded with K2PtC16. The reason, in our opinion, was that the configuration of K2PtCI6 consisted of cubical octahedra, but Pt(NH3)2CI 2 had a plane quadrilateral configuration which could be distorted easily; so that Pt(NH3)2C12 could enter the channel of the zeolite, but the K2PtCI6 could only partly enter it; which was proved with 129Xe-NMR experiments by Yang et al. [9]. The different location of the two Pt compounds on zeolite affected the disperion of Pt. For example, no Pt particle larger than 4 nm was observed on the TEM pictures of B aKL loaded with Pt (NI-I3 )2C12, and most of the Pt particles had a size in the range 2-3 nm, but on the BaKL loaded with KzPtC16, the ratio of Pt
Jia-Lu Dong et al. / Applied Catalysis A." General 112 (1994) 105-115
109
Table 2 Catalytic properties of BaKL and BaKY zeolites loaded with K2PtCI6 or Pt(NH3)2CI 2 Soaking solution
BaKL
K2PtC% Pt(NH 3)2C12
BaKY
Conv.(%)
Selec.(%) a
Conv.(%)
Selec.(%) a
23.5 99.4
65.8 93.1
I 1.4 52.4
49.5 63.1
~The selectivity for benzene in the n-hexane aromatization.
particles with a size larger than 4 nm was found to be 36.5%. As a result, different catalytic properties existed on these Pt supported zeolites with these two kinds of Pt compounds.
3.2. The function of secondary pores in zeolites Sample KL-I was not washed neutral but kept at strong basic conditions ( p H = 13) in the synthesis process; then it was ion exchanged with Ba 2+ and loaded with Pt(NH3)2C12 as described before. In contradiction to that reported by Mielazarski et al. [ 5 ], the Pt/BaKL-I zeolites kept their high benzene selectivities (more than 94%) at different n-hexane conversion in aromatization of n-hexane (Table 3). Besides, 0.4% Pt/BaKL-I catalyst was found to retain as excellent aromatization properties as those of 0.6% Pt/BaKL (Fig. 2). Different from BaKL zeolite, a new peak with a 20 value of 23.5 appeared on the XRD pattern of BaKL-I, and according to ASTM [ 10] it should be attributed to BaSiO3. Chemical analysis showed that the ratio of BaO/A1203 was higher on BaKL-I than on BaKL (Table 4) which resulted from the excess BaSiO3, and Ba(OH)2 etc. existed on the BaKL-I sample. The adsorption isotherm of benzene on the BaKL-I sample was found to be a typical Langmuir isotherm, without capililary condensation at higher P/Po as found in that of the BaKL sample (Fig. 3); which revealed that impurities such as BaSiO3 had blocked the secondary pores Table 3 Conversion and product distribution from reaction of n-hexane on Pt/BaKL-I samples Reaction conditions: 763 K, WHSV = 2.20 h-% 5 h Amount of Pt loaded Conversion of n-hexane Select. to Benzene (wt.%) (mol-%) (%)
Select. to C~-C5 (%)
Select. to C6 isomers (%)
0.2 0.3 0.4 0.6 0.8 1.1
2.3 2.7 2.2 2.6 1.7 2.1
3.5 1.3 0.1 0.4 0.1 0.2
76.1 83.2 94.2 91.0 96.1 95.2
94.2 96.0 97.7 97.0 98.2 97.7
110
Jia-Lu Dong et al. / Applied Catalysis A: General 112 (1994) 105-115
loo
// E 8o-
8
/
>~ 6 0
~ 4o I
c
it
/: /
20-
i
l
02
l
I
I
0.4 06 0.8 1.0 Amount of Pt loaded ( w t . - * / , )
Fig. 2. Conversion of n-hexane on (A) Pt/BaKL-I and (B) Pt/BaKL zeolites in aromatization (768 K) vs. amount of platimum loaded. Table 4 Chemiscal analysis data of L zeolites Sample
SIO2/A1203
K20/AIzO3
BaO/AI203
BaKL BaKL-I
5.40 5.81
0.80 0.92
0.30 0.53
22"
i
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o
.o~
t~.-
~
. . . .
-
6 E <
i
i
i
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i
0.8 P I Po
Fig. 3. Adsorption isotherms of benzene on L zeolites. ( X ) KL, (o) KI-I, (o) Linde-KL, ( • ) BaKL, (z~) BaKL-I.
and part of the channels of BaKL-I, which made the capillary condensation disappear and lowered the adsorption capacity. This blocking was found to decrease not only the Pt consumed and the adsorption capacity, but also some side-reaction occurred in the secondary pores. This conclusion was confirmed by comparing Pt/ KL and Pt/KL-I catalysts. It was certain that the strong basic condition of the
Jia-Lu Dong et al. / Applied Catalysis A: General 112 (1994) 105-115
111
BaKL-I and KL-I samples formed had some influences on their dispersion and catalytic activity of the supported Pt, and further investigations are needed.
3.3. Influence of acidic-basic properties of zeolites NH3-TPD and CO2-TPD were used to determine the acidic-basic properties of NHaL and KL (Table 5). Two peaks were found in NH3-TPD of NHaL, as compared to only one low-temperature peak in that of KL zeolite, showing that the acidic strength of NHaL was greater. In the meantime, the total acidic amount of NH4L was 4.5 times larger than that of KL zeolite. NHnL zeolite exhibited strong BrCnsted acidity, as characterized by the 1540 c m - 1 band in the FT-IR spectrum of adsorbed pyridine. After being loaded with 0.6% of Pt on NHaL, the area of the 1540 c m - 1 band in the FT-IR spectrum was reduced (Fig. 4), which meant that loading of Pt lowered the BrCnsted acidity of NH4L zeolite. Isopropanol decomposition, which could be dehydrated on the acid site or dehydrogenated over the basic site [ 11 ], was employed as a probe reaction. Hydrogen was chosen as the carrier gas both in this reaction and in the aromatization, in order to correlate the catalytic behaviours of supported Pt samples in these two reactions. Isopropanol decomposition on NHaL zeolite only produced water and propene. At 498 K, almost all of the reactant had been converted. KL showed some basicity in the probe reaction and aceton formed on it. A1498 K, the isopropanol conversion and acetone selectivity on KL zeolite were 11.9% and 41.3%, respectively. The Pt loaded suppressed dehydration, but accelerated dehydrogenation of isopropanol over KL zeolite (the conversion and acetone selectivity on Pt/NH4L were 49.2% and 94.2%), which indicated the dehydrogenation function of the Pt cluster. In this reaction, Pt/NH4L, which showed a strong acidity, enhanced the conversion of isopropanol to form water and propene. It is known that on acidic centers of zeolites, toluene alkylation with methanol produced xylene, and on basic sites ethylbenzene formed [11,12]. In toluene alkylation with methanol at 693 K, the main product on NH4L zeolite was xylene, as expected, but the yield of xylene was only 1.3%. Xylene was found to be the main product over Pt/NH4L zeolite, in such alkylation, and the yield of xylene was 5.7%. It proved further that Pt loading considerably enhanced the acidic catalysis of NHaL zeolite. Table 5 TPD results of KL and NH~L zeolites CO2-TPD
NH3-TPD Samples
KL
NI-14L
KL
NH4L
Tm(K) Amount ( m m o l / g )
392 0.63
394, 656 1.12, 1.72
426 14.4.10 -2
426 5.31.10 -2
112
Jia-Lu Dong et al. / Applied Catalysis A: General 112 (1994) 105-115
The Pt/KL was also sensitive for thiophene poisoning in isopropanol decomposition: only 0.02 mmol/g of thiophene decreased acetone yield by 25% at 593 K. However, poisoning with 0.17 mmol/g thiophene only decreased the water yield by one fifth in isopropanol decomposition over Pt/NH4L at 443 K. The main active centres of Pt/NH4L zeolite in this probe reaction were found to be the acid sites, and just as those of NHaL, they were easily poisoned by pyridine (Fig. 5). Owing to the strong acidity, the NH4L zeolite deactivated quickly in isopropanol decompositon at 443 K, even in hydrogen atmosphere. After 5 h on steam, the
A
1560
i
i
i 1500 cm_iI 14 40
Fig. 4. FT-IR spectra of adsorbed pyridine on L zeolites before and after loading of platinum. (A) NH4L, purged at 423 K, (B) Pt/NFL,L, purged at 423 K.
. 0.8 ~ ._> i:i ._u
~'
o.4
o
0
~
0.026
0.052
Amountof pyrldine(rnmol/g)
Fig. 5. Influence of pyridine poison on the dehydration of isopropanol on Pt/NH4L zeolite.
Jia-Lu Dong et aL /Applied Catalysis A: General 112 (1994) 105-115
100 [
._~
~
~
70
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2
~
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3 4 5 6 Reaction t~me (h) Fig. 6. Reactivity of NH4L zeolites before and after loading of platinum in the decomposition of isopropanol (443 K) vs. reaction time. (A) Pt/NH4L zeolite in hydrogen, (B) Pt/NHnL zeolite in helium, (C) NH4L in zeolite in hydrogen.
conversion of isopropanol on NH4L decreased from 100% to 66.2%, and 2.7% coke formed on it. Oppositely, Pt/NH4L zeolite always kept its high activity under the same reaction conditions. The conversion of isopropanol was still 100% after use in steam for 6 h, and only 1.6% of coke formed. When the reaction carrier gas was changed from hydrogen to helium, the isopropanol conversion on Pt/NH4L decreased clearly from 100% to 58.1% after 5 h, as seen in Fig. 6, and 4.3% of coke formed on the sample. Similar to that observed by Chow et al. [ 13], these results showed the generally accepted scheme that Pt held the acid sites clean, i.e. coke precursors on the acid sites were removed by interacting with hydrogen. In the reaction atmosphere of helium, hydrogen no longer existed in the gas phase and the Pt cluster on NH4L could not catalyze dehydrogenation of isopropanol, so that no hydrogen formed for the Pt cluster to help the acid sites of the zeolite to remove the coke. The catalytic properties of Pt dispersed on acidic NH4L were totally different from that on basic KL zeolite. In the aromatization of the n-hexane at 768 K, Pt/ KL exhibited an exceptional selectivity of benzene (93.0%), similar to that of Pt/ BaKL (92.7%), and was very sensitive to thiophene poison. It was thus shown that both P t / K L and Pt/BaKL zeolites were monofunctional catalysts for aromatization. Pt/NH4L zeolite was proven to be a bifunctional catalyst, since in aromatization of hexane, more isomers formed on it htan aromatic products. Besides, poison with 0.12 mmol/g of thiophene decreased the n-hexane conversion over Pt/BaKL zeolite from 93.1% to 8.5%, and consequently no benzene formed at all; the same amount of thiophene, however, could not lower the benzene selectivity on Pt/NH4L zeolite in the same reaction conditions. The main CO band observed on Pt/NH4L zeolite was seen (Fig. 7) after adsorption in 2069 c m - i for linear CO, but in 2034 c m - ] with a shoulder band of 2018
114
Jia-Lu Dang et al. /Applied Catalysis A: General 112 (1994) 105-115
2140
21100
i
2060
I
2020
i
1980 cm-1
1940
Fig. 7. FT-IR spectra of carbon monoxide adsorbed on (A) Pt/NI-I4L and (B) Pt/KL zeolites.
c m - ~ on P t / K L zeolite. The difference in the electronic state of the Pt cluster was found to be the main factor causing the change of Vco on Pt supported L zeolites. It was reported by Primet et al. that in Pt/A1203 [ 14] or Pt/faujasite [ 15], a Vco vibration free of C O - C O coupling could be reached closed to 2052 c m - '. For Pt/ NH4L zeolite, the Uco value was higher than in this reference, which indicated a decrease in back donation from Pt to CO due to the Pt-zeolite interaction, since the Pt cluster interacted with the electron-acceptor acidic sites and showed a kind of "electron deficiency". In P t / K L zeolite, the CO wavenumber was lower than the reference at 2052 c m - ], which could be explained as an increase in back-donation from Pt to CO, resulting from the Pt interacting with electron donor framework atoms of the KL zeolite, such as the negatively charged lattice oxygen. Those basic sites would play the same role as electon donor [ 16] and lower Vco of CO adsorbed on Pt/KL. It is known that the anti-sulfur properties of supported Pt catalysts was proportional to the "electron deficiency" of the Pt particles [17], and Pt/NH4L zeolite exhibited a higher resistance to thiophene poison than Pt/KL in both isopropanol decomposition and aromatization; so it was proven that the Pt active centres on Pt/NH4L zeolite unlike those on P t / K L with electron excess, showed obvious "electron deficiency". The different acidic-basic properties of the zeolite changed the electronic state of the supported Pt and caused the different catalytic properties of them.
4. Concluding remarks Some conclusions emerged from this work: the pore structure of the zeolite not only affected the sizes and location of the Pt particles, but also provided hindrance for the intermediates or products to form selectively in the reactions. The pore system like zeolite L, in our opinion, was beneficial for highly dispersed Pt clusters
Jia-Lu Dong et al. / Applied Catalysis A: General 112 (1994) 105-115
115
to exhibit an excellent catalytic activity in aromatization. The acidic-basic properties of zeolites had strong effects on the electron state and location as well as the catalytic properties of Pt active centres.
Acknowledgements Supports of this work by the National Natural Science Foundation of China, and the Chinese Education Committee were greatfully acknowledged.
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