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42 Hydroisomerization of tetralin on zeolite beta: Influence of crystal size
Science and Technology in Catalysis 2002 Copyright 9 2003 by Kodansha Ltd.
42 Hydroisomerization of Tetralin on Zeolite Beta: Influence of Crystal Size
Praveen Sharma, Yasutoshi Iguchi, Yasushi Sekine, Eiichi Kikuchi, and Masahiko Matsukata* Department of Applied Chemistry, Waseda University, Tokyo 169-8555, Japan.
Abstract Though light cycle oils (LCO) obtained in the fluid catalytic cracking (FCC) are rich in aromatics and sulphur but are insufficient to diesel fuels due to the low cetane number. It is worthwhile to produce alkylbenzenes from LCO without producing small olefinic hydrocarbons through hydrotreating polyammatic compounds present in the LCO. In the present study tetralin was used as a model compound. Reactions such as ring opening, dealkylation, transalkylation, hydrogen transfer and coke formation were studied using Platinum-supported beta as catalyst. The objective of this study was to investigate the relationship between the crystal size of beta zeolite and catalytic properties for hydro treatment of tetralin. Key words: Zeolite beta, Light cycle oil (LCO), Tetralin, Crystal size 1. INTRODUCTION The fractionally distillation of crude oil mainly produces gasoline, naphtha, kerosene, light oil, vacuum gas oil and residual oil. The light cycle oil (LCO) obtained from the fluid catalytic cracking (FCC) of residual oils and naphtha, has a low cetane number and high contents of aromatics and sulphur. Out of the 62 % of aromatics present in the LCO, about 30 % are 2ring aromatics such as Naphthalene. Zeolites are used in various catalytic reactions [1] for example alkylation, cracking, isomerization, etc. due to their activity, selectivity, thermal stability, and reusability and ecofriendly nature. Amongst the few isolated reports [2-4] on treating of LCO using zeolites include study by Corma et. al. which suggested beta as a suitable catalyst for gas-oil cracking reactions [2]. Zeolite beta has 12-membered ring opening and three-dimensionally connected micropore system. Such structural features offer attractive characters as a catalyst in addition to possible synthesis in a wide range of Si/2AI ratios. In the present study beta with different crystal sizes and similar acid strengths were synthesized with a "Steam-assisted conversion (SAC)" technique. It will be shown that zeolite beta crystals can be optimized by changing parent hydrogel composition and can be utilized to produce commercially important alkylbenzenes such as methyl indane.
2. EXPERIMENTAL
2.1. Synthesis
219
220 P. Sharma et
al.
Three different beta zeolite samples were synthesized by the SAC method. Details about the SAC technique can be obtained elsewhere [5,6]. Initial gel compositions: (A) 1 AlzO3: 50 SiO2:3.0 NaOH: 15 TEAOH: 500 H20, (B) 1 A1203:50 SiO2:2.4 NaOH: 15 TEAOH: 500 HzO, and (C) 1 A1203:50 SiO2:1.2 NaOH: 7.5 TEAOH: 500 H20. The temperature and time of crystallization for all the samples were 165~ for 24 h. Aluminium hydroxide and fumed silica were used as sources for A1 and Si. H-form of the samples was obtained by ion exchange of assynthesized materials with NH4NO3 at 80~ for 4 h, followed by thermal treatment at 500~ for 24 h in 20% O2 atmosphere. The as-synthesized materials were characterized by using XRD, FE-SEM, TPD of ammonia, N2 adsorption, chemical analysis and FT/IR spectra using pyridine as probe molecule.
2.2. Platinum loading Ion-exchange method was used to load 1 wt% of Pt on all zeolite beta materials prior to catalytic experiments. [Pt(NH3)4]CI2.HzO was used as source of platinum. A desired amount of platinum source was transferred to a beaker and ammonia soln. was added till the pH of the resultant solution became 10. In the next step zeolitewas added to this solution and resultant mixture was stirred for about 24 h at room temperature. Finally the Pt-supported zeolite was filtered out from the solution using vacuum filter, washed with distilled water several times and dried at 100~
2.3. Catalytic reactions Catalytic experiments were performed on a specially-designed continuous flow, pressurized fixed-bed reactor. The reactor consisted of a Stainless-steel tube, 25 cm long and 1 em in diameter covered with coil heater regulated by a thermal controller. About 0.2 g of Ptsupported zeolite beta sample was packed in between two plugs of glass wools in the center of the reactor. Reaction was carried out under H2 flow. Gas chromatograph (SHIMADZU GC-8A) equipped with a TC-1 capillary column and integrator (CR 8A), analyzed reaction products. The catalytic tests were carded out under following conditions; reaction temperature = 250-350 ~ Pressure = 1.1Mpa, W/F = 2.82x 102 g-min/mol, and reactant feed rate = 7.09x10 "4mol/min.
Fig. 1. Scanning electron micrographs of as-synthesized zeolite beta. Scale bar 300 nm. Table 1. Physicochemical characteristics of as-synthesized zeolite beta samples Sample
Na20/AI203 b
TEA +/uc a
Si/2A1 b
BET m2/g
A
0.47
4.70
48
244
B
0.57
4.47
49
136
C
0.00
4.43
44
42
aCI-IN an~dysis, b Chemical analysis by m e ~ s of IcP
221 3. RESULTS AND DISCUSSION The physicochemical characteristics of the as-synthesized zeolite beta samples are summarized in Table 1. FE-SEM images are shown in Figure 1. It was possible to control the sizes of crystals simply by changing the initial reactant compositions such as amount of alkali (sodium) and template (Tetraethylammonium hydroxide), while keeping Si/2A1 ratio the same. Figure 2 shows the XRD patterns of the as-synthesized zeolite beta. Pure beta phase is obtained for all the samples. The intensity of the XRD patterns increased with increasing crystal size, which was expected.
j ~
~2
# B
.
.
.
.
.
I
,
,
5
20 40 2 0 (CuKet) / Degree Fig. 2. XRD patterns of as-synthesized zeolite beta with different crystal sizes.
1400 1800 1600 Wavenumber / cm ": Fig. 3. FT-IR of the H-form beta samples: pyridine adsorption region.
Acid site distribution of H-beta samples was studied by FT-IR after adsorbing pyridine, as shown in Figure 3. The distribution of Br6nsted type (1540-50 cm 1) and Lewis type acid sites (1440 --1450 cm 1) was almost similar for all three samples. Figure 4 shows the catalytic activity and the product distribution on Pt-supported beta zeolite with different crystal sizes. The tetralin conversion was decreased with decreasing crystal size. Pt supported on H-beta with an average crystal size of about 600 nm having high micropore area and low external surface area showed high yield and selectivity for isomerization reactions and low cracking. Pt support on zeolite is important for the process as Pt enhances isomerization (zeolite surface) and suppresses cracking (inside micropore). Table 2. Micropore Analysis of Pt-supported H-beta and coke deposition Sample
Coke deposition Micro-pore volume Micropore Area External surface area (mg -c .ar.bon/g-_catalyst) (cc/g) (mZ/g) (mZ/g) ......
A
0.15
279
288
C
0.14
509
46
148 62.9
Large size crystals of beta zeolite are relatively stable [3,8] and coke deposition is low compared to smaller size beta crystals (as shown in table 2), which are unstable to the conventional calcination procedures that cause severe agglomeration of small crystallites.
222 P. Sharma et
al.
However crystal size has to be optimized, as too large a crystal size while being more thermally stable, are less active and selective due to higher diffusion limitations, as has been shown to occur on zeolite Y [8]. C 7 c0mpounds C7
C ~(::i:i:
'
C9
:i:il.
.... "
I
I#
i
Clo
Cn
~
~i_--_--:'[!/
""
""
C9comp ounds
~'-"~'"
I
I
I
tl
I
I# t f
I
I
II
I
'
'""
'
'
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C 10 compounds
I
I l
C 11 compounds
A
I
I
l
9
~
I
0
I
l
l
: ~ T
A
/
T
45 Conversion (%)
Fig. 4. Conversion of tetralin and product distribution over Pt supported on beta zeolites with different crystal sizes; Temperature 300 ~ 1.1 Mpa. 4. CONCLUSION Crystal size of beta zeolite can be tailored by "Steam-assisted crystallization (SAC)" method. Pt supported on large crystal size H-beta showed a high activity for hydro treatment of tetralin because of low surface area, high micropore area and low coke deposition. REFERENCES
1. B. Halgeri and J. Das. Appl. Catal. A. 181 (1999), 347. 2. L. Bonetto, M.A. Camblor, A. Corma and J. Perez-Pariente. Appl. Catal. A. 82 (1992) 37. 3. Corma, V.Fornes, F.Melo and J.Perez-Pariente, ACS Symp.Ser., 375 (1998) 49. 4. Ch.Marcilly, J.Deves and F.Raatz, Eur.Pat.Appl.,EP 278 (1998) 839. 5. P.R. Had Prasad Rao, M. Matsukata, Chem. Commun., (1996) 1441. 6. M. Matsukata, M. Ogura, T. Osaki, P.R.H.P. Rao, M. Nomura and E. Kikuchi. Top. in Catal. 9 (1999) 77. 7. W.C. Cheng, A. W. Peters, K. Rajagopalan, irr H. J. Lovink and L.A. Pine (Editors), The Hydrocarbon chemistry of FCC Naptha Formations Editions Technip, Pads, (1990), 105. 8. M.A. Camblor, A. Corma, A. Martinez, F. A. Mocholi and J. Perez-Pariente, Appl. Catal., 55 (1989) 65.
42 Hydroisomerization of Tetralin on Zeolite Beta: Influence of Crystal Size
Praveen Sharma, Yasutoshi Iguchi, Yasushi Sekine, Eiichi Kikuchi, and Masahiko Matsukata* Department of Applied Chemistry, Waseda University, Tokyo 169-8555, Japan.
Abstract Though light cycle oils (LCO) obtained in the fluid catalytic cracking (FCC) are rich in aromatics and sulphur but are insufficient to diesel fuels due to the low cetane number. It is worthwhile to produce alkylbenzenes from LCO without producing small olefinic hydrocarbons through hydrotreating polyammatic compounds present in the LCO. In the present study tetralin was used as a model compound. Reactions such as ring opening, dealkylation, transalkylation, hydrogen transfer and coke formation were studied using Platinum-supported beta as catalyst. The objective of this study was to investigate the relationship between the crystal size of beta zeolite and catalytic properties for hydro treatment of tetralin. Key words: Zeolite beta, Light cycle oil (LCO), Tetralin, Crystal size 1. INTRODUCTION The fractionally distillation of crude oil mainly produces gasoline, naphtha, kerosene, light oil, vacuum gas oil and residual oil. The light cycle oil (LCO) obtained from the fluid catalytic cracking (FCC) of residual oils and naphtha, has a low cetane number and high contents of aromatics and sulphur. Out of the 62 % of aromatics present in the LCO, about 30 % are 2ring aromatics such as Naphthalene. Zeolites are used in various catalytic reactions [1] for example alkylation, cracking, isomerization, etc. due to their activity, selectivity, thermal stability, and reusability and ecofriendly nature. Amongst the few isolated reports [2-4] on treating of LCO using zeolites include study by Corma et. al. which suggested beta as a suitable catalyst for gas-oil cracking reactions [2]. Zeolite beta has 12-membered ring opening and three-dimensionally connected micropore system. Such structural features offer attractive characters as a catalyst in addition to possible synthesis in a wide range of Si/2AI ratios. In the present study beta with different crystal sizes and similar acid strengths were synthesized with a "Steam-assisted conversion (SAC)" technique. It will be shown that zeolite beta crystals can be optimized by changing parent hydrogel composition and can be utilized to produce commercially important alkylbenzenes such as methyl indane.
2. EXPERIMENTAL
2.1. Synthesis
219
220 P. Sharma et
al.
Three different beta zeolite samples were synthesized by the SAC method. Details about the SAC technique can be obtained elsewhere [5,6]. Initial gel compositions: (A) 1 AlzO3: 50 SiO2:3.0 NaOH: 15 TEAOH: 500 H20, (B) 1 A1203:50 SiO2:2.4 NaOH: 15 TEAOH: 500 HzO, and (C) 1 A1203:50 SiO2:1.2 NaOH: 7.5 TEAOH: 500 H20. The temperature and time of crystallization for all the samples were 165~ for 24 h. Aluminium hydroxide and fumed silica were used as sources for A1 and Si. H-form of the samples was obtained by ion exchange of assynthesized materials with NH4NO3 at 80~ for 4 h, followed by thermal treatment at 500~ for 24 h in 20% O2 atmosphere. The as-synthesized materials were characterized by using XRD, FE-SEM, TPD of ammonia, N2 adsorption, chemical analysis and FT/IR spectra using pyridine as probe molecule.
2.2. Platinum loading Ion-exchange method was used to load 1 wt% of Pt on all zeolite beta materials prior to catalytic experiments. [Pt(NH3)4]CI2.HzO was used as source of platinum. A desired amount of platinum source was transferred to a beaker and ammonia soln. was added till the pH of the resultant solution became 10. In the next step zeolitewas added to this solution and resultant mixture was stirred for about 24 h at room temperature. Finally the Pt-supported zeolite was filtered out from the solution using vacuum filter, washed with distilled water several times and dried at 100~
2.3. Catalytic reactions Catalytic experiments were performed on a specially-designed continuous flow, pressurized fixed-bed reactor. The reactor consisted of a Stainless-steel tube, 25 cm long and 1 em in diameter covered with coil heater regulated by a thermal controller. About 0.2 g of Ptsupported zeolite beta sample was packed in between two plugs of glass wools in the center of the reactor. Reaction was carried out under H2 flow. Gas chromatograph (SHIMADZU GC-8A) equipped with a TC-1 capillary column and integrator (CR 8A), analyzed reaction products. The catalytic tests were carded out under following conditions; reaction temperature = 250-350 ~ Pressure = 1.1Mpa, W/F = 2.82x 102 g-min/mol, and reactant feed rate = 7.09x10 "4mol/min.
Fig. 1. Scanning electron micrographs of as-synthesized zeolite beta. Scale bar 300 nm. Table 1. Physicochemical characteristics of as-synthesized zeolite beta samples Sample
Na20/AI203 b
TEA +/uc a
Si/2A1 b
BET m2/g
A
0.47
4.70
48
244
B
0.57
4.47
49
136
C
0.00
4.43
44
42
aCI-IN an~dysis, b Chemical analysis by m e ~ s of IcP
221 3. RESULTS AND DISCUSSION The physicochemical characteristics of the as-synthesized zeolite beta samples are summarized in Table 1. FE-SEM images are shown in Figure 1. It was possible to control the sizes of crystals simply by changing the initial reactant compositions such as amount of alkali (sodium) and template (Tetraethylammonium hydroxide), while keeping Si/2A1 ratio the same. Figure 2 shows the XRD patterns of the as-synthesized zeolite beta. Pure beta phase is obtained for all the samples. The intensity of the XRD patterns increased with increasing crystal size, which was expected.
j ~
~2
# B
.
.
.
.
.
I
,
,
5
20 40 2 0 (CuKet) / Degree Fig. 2. XRD patterns of as-synthesized zeolite beta with different crystal sizes.
1400 1800 1600 Wavenumber / cm ": Fig. 3. FT-IR of the H-form beta samples: pyridine adsorption region.
Acid site distribution of H-beta samples was studied by FT-IR after adsorbing pyridine, as shown in Figure 3. The distribution of Br6nsted type (1540-50 cm 1) and Lewis type acid sites (1440 --1450 cm 1) was almost similar for all three samples. Figure 4 shows the catalytic activity and the product distribution on Pt-supported beta zeolite with different crystal sizes. The tetralin conversion was decreased with decreasing crystal size. Pt supported on H-beta with an average crystal size of about 600 nm having high micropore area and low external surface area showed high yield and selectivity for isomerization reactions and low cracking. Pt support on zeolite is important for the process as Pt enhances isomerization (zeolite surface) and suppresses cracking (inside micropore). Table 2. Micropore Analysis of Pt-supported H-beta and coke deposition Sample
Coke deposition Micro-pore volume Micropore Area External surface area (mg -c .ar.bon/g-_catalyst) (cc/g) (mZ/g) (mZ/g) ......
A
0.15
279
288
C
0.14
509
46
148 62.9
Large size crystals of beta zeolite are relatively stable [3,8] and coke deposition is low compared to smaller size beta crystals (as shown in table 2), which are unstable to the conventional calcination procedures that cause severe agglomeration of small crystallites.
222 P. Sharma et
al.
However crystal size has to be optimized, as too large a crystal size while being more thermally stable, are less active and selective due to higher diffusion limitations, as has been shown to occur on zeolite Y [8]. C 7 c0mpounds C7
C ~(::i:i:
'
C9
:i:il.
.... "
I
I#
i
Clo
Cn
~
~i_--_--:'[!/
""
""
C9comp ounds
~'-"~'"
I
I
I
tl
I
I# t f
I
I
II
I
'
'""
'
'
" ','
C 10 compounds
I
I l
C 11 compounds
A
I
I
l
9
~
I
0
I
l
l
: ~ T
A
/
T
45 Conversion (%)
Fig. 4. Conversion of tetralin and product distribution over Pt supported on beta zeolites with different crystal sizes; Temperature 300 ~ 1.1 Mpa. 4. CONCLUSION Crystal size of beta zeolite can be tailored by "Steam-assisted crystallization (SAC)" method. Pt supported on large crystal size H-beta showed a high activity for hydro treatment of tetralin because of low surface area, high micropore area and low coke deposition. REFERENCES
1. B. Halgeri and J. Das. Appl. Catal. A. 181 (1999), 347. 2. L. Bonetto, M.A. Camblor, A. Corma and J. Perez-Pariente. Appl. Catal. A. 82 (1992) 37. 3. Corma, V.Fornes, F.Melo and J.Perez-Pariente, ACS Symp.Ser., 375 (1998) 49. 4. Ch.Marcilly, J.Deves and F.Raatz, Eur.Pat.Appl.,EP 278 (1998) 839. 5. P.R. Had Prasad Rao, M. Matsukata, Chem. Commun., (1996) 1441. 6. M. Matsukata, M. Ogura, T. Osaki, P.R.H.P. Rao, M. Nomura and E. Kikuchi. Top. in Catal. 9 (1999) 77. 7. W.C. Cheng, A. W. Peters, K. Rajagopalan, irr H. J. Lovink and L.A. Pine (Editors), The Hydrocarbon chemistry of FCC Naptha Formations Editions Technip, Pads, (1990), 105. 8. M.A. Camblor, A. Corma, A. Martinez, F. A. Mocholi and J. Perez-Pariente, Appl. Catal., 55 (1989) 65.