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Low temperature hydrocracking of paraffinic hydrocarbons over hybrid catalysts
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
1005
Low temperature hydrocracking of paraffinic hydrocarbons over hybrid catalysts I. Nakamura, K. Sunada and K. Fujimoto Depamnent of Applied Chemistry, Faculty of Engineering, The University of Tokyo 7-3-1, Hongo, Bunkyo-ku, Tokyo 113 Japan
A hybrid catalyst, which was prepared by physical mixing of a H-ZSM-5 and Pd/SiO2. showed an excellent activity for the hydrocracking of n-paraffins at low reaction temperature (503 K). In the n-heptane cracking, the hybrid catalyst gave only isomerized heptane and propane and equimolar amount of i-butane whereas the products on H-ZSM-5 alone distributed from C3 to C9 and C4 products contained all kind of paraffins and olefins. The wide product distribution for H-ZSM-5 system should be attributed to the reaction path comprising oligomerization and cracking of the olig0mer. The simple products for the H2-hybrid system should be formed through no other reaction path than the primary cracking reaction on H-ZSM-5, which proceeds through a intermediate containing the protonated dialkylcyclopropane structure.
I. INTRODUCTION Hydrocracking of petroleum heavy hydrocarbons have been practiced extensively commercially in petroleum refining to produce high quality gasoline, jet fuel, gas oil and lubricants. Many hydrocracking catalysts of commercial importance are dual functional catalysts containing both hydrogenation components such as sulfided Ni-Mo or Ni-W and acidic components such as zeolites. The most predominant reaction mechanism for the hydrocracking of alkane is as follows: (1) the dehydrogenation of alkane to alkene on the supported metal; (2) proton addition to the alkene to form carbenium ion on the acidic component; (3) B-scission of the carbenium ion to form smaller carbenium ion and alkene on the acid component; (4) hydrogenation of the cracked alkene to alkane on the supported metal [1]. On the other hand, it was proposed that acid catalyzed reactions such as skeletal isomerization of paraffins [2] and disproportionation [3] over metal supported acid catalysts were promoted by spillover hydrogen (proton) on the acid catalysts. Hydrogen spillover phenomenon from noble metal to other component at room temperature has been reported in many cases [4, 5]. In the present work, hydrocracking at low reaction temperature was studied using hybrid catalysts containing H-ZSM-5 zeolite and a supported noble metal from the standpoint of hydrogen spillover.
1006
2. EXPERIMENTAL 2.1
Catalyst preparation Zeolites used were a H-ZSM-5 (Toso, HSZ-840NHA SIO2/A1203ratio=44.0) and USY zeolite (Catalyst & Chemicals Ind. SiO2/A1203 ratio=8.6 ). Silica-supported palladium was prepared by impregnating a commercially available SiO2 (Aerosil 380, BET specific surface area 380 m2/g) with PdC12 from its aqueous hydrochrolic solution which was followed by the calcination in air at 723 K for 3 h and the reduction in flowing hydrogen at 723 K for 1 h. Hybrid catalyst was prepared by co-grinding the mixture of 4 weight parts of the H-ZSM-5 with one weight part of Pd/SiO2 (2.5wt%) and pressure molding the mixture to granules to 20/40 mesh. Catalysts were activated in air at 723 K for 2 h and reduced in flowing hydrogen at 673 K for lh, before use. 2.2 Reaction apparatus and procedure The hydrocracking of n-paraffins was conducted with a continuous flow type fixed bed reaction apparatus under pressurized conditions. The reactor was a stainless steel tube with an inner diameter of 6 mm. The feed material which had been deeply desulfurized was fed by a liquid pump. The mole ratio of H2/n-CTH16 or N2/n-C7H16 in the feed was 9:1. Products were withdraw in the gaseous state and analyzed by a capillary gas chromatograph.
3. RESULTS AND DISCUSSION
3.1.
n-C7 cracking on Pd-based ZSM-5 catalysts Hydrocracking of n-heptane, which should give only C3 and C4 hydrocarbons in acid catalyzed cracking was studied. Figure 1 shows the changes of catalytic activities of a variety of catalysts containing Pd/SiO2 and/or H-ZSM-5 for n-heptane hydrocracking as a function of reaction time. The catalytic activity of H-ZSM-5 was not affected by the atmosphere and decreased quickly. Pd/SiO2 showed little activity for both dehydrogenation and cracking of n-CTH16. On the other hand, the catalytic activity of a hybrid catalyst comprising Pd/SiO2 and H-ZSM-5 was the highest and its activity was kept constant under hydrogen atmosphere while it was much lower and decreased quickly under nitrogen atmosphere. This phenomenon clearly shows that the presence of hydrogen is essential in order to generate hydrocracking activity. It is well known that the supported platinum shows a high catalytic activity for the dehydrogenation of alkane whereas the supported palladium does not. The results shown in Figure 1 suggest that the dehydrogenation activity of supported metal is not essential for the appearance of the alkane hydrocracking activity. The essential point is that the hydrogen-activating component intimately contacts with acidic catalyst. The present authors have pointed out that the skeletal isomerization of lower alkane is effectively promoted by the hybrid catalyst composed of Pd/SiO2 or Pt/SiO2 and H-ZSM-5 and that the hydrogen spillover is the key step of isomerization reaction. In the present case also, the hydrogen migration from Pd/SiO2 to H-ZSM-5 should be essential for the high and stable catalytic activity. The characteristic feature of the product distribution is that the reaction products of H2-hybrid catalyst system are only isomerized heptane and propane and equimolar amount of
1007 isobutane (little n=C4HI 0 was formed), whereas the products on H-ZSM,5 alone distributed from C3 to C9 and C4 products contained all kind of paraffins and olefins as shown in Figure 2. The wide product distribution for H-ZSM-5 system should be attributed to the reaction path comprising polymerization and cracking. In the hydroisomerization of n-pentane over hybrid catalyst containing H--ZSM--5 and supported noble metal catalyst, it was proposed that a hydrogen molecule spills over on to zeolite surface as a proton and a hydride [2]. The proton promotes cracking reaction even at low reaction temperature. The hydride generated simultaneously stabilizes intermediate carbenium ion to prevent oligomerization and cracking of oligomer and promote isomerization of alkane. The simple products for the H2-hybrid system should be formed through no other reaction path than the primary cracking reaction on H-ZSM-5. 70 ~ 60 -
60
Hybrid(H2)
[[I •
Hybrid(H 2)
50-H.ZSM.5(H2 )
~50 -
~4o
c
o 40 -
.m
30 r-o ~~
-
H-ZSM-5(H 2)
~2o
O 20 -
10
I
10 Pd!Si02(H2) . . . . -
0
-
I
o~
I
0.5 1 1.5 2 Time on Stream(h)
2.5
~
1 2 3 4 5 6 7 8 9 1011 12 Carbon Number
503 K, H2 or N2/n-C7--9, 1.0 MPa, Pd/SiO2:H-ZSM-5=I:I, W/F=I.2 g h mol-1 Figure 1. Hydrocracking of n-heptane with ZSM-5 catalysts.
Figure 2. C-number distribution. The same experiments as Figure 1 (TOS=I h).
Hybrid I-I2 ........................................................... Hybrid N2 ~....:~....: 4 ~ i ]
C~
n-C4
|
H-ZSM-5 H lltl~Qtm~l~R..:.~ H-ZSM-5 N2 ~
~
i
i
I
I
I
I
I
I
I
I
0
10
20
30
40
50
60
70
80
I 90 100
Distribution (mol%) Figure 3. Distribution of C4 hydrocarbons formed in the hydrocracking of n-C7. The same experiments as Figure 2.
1008 In hydrocracking of normal paraffin with metal supported acid catalyst, the iso/normal ratios in the paraffinic products generally exceed the thermodynamic equilibrium. It proves that at least some of the branched paraffins are primary products of the cracking and not a results of the post isomerization. This is particularly true in the case of C4, since n-butane cannot be isomerized under typical hydrocracking conditions with a zeolite catalyst. Especially the fact that isobutane is the sole C4 product in the present system as shown in Figure 3 suggests that the hydrocracking on the hybrid catalyst should not proceed through the cracking of n-paraffin to form smaller n-paraffin and another n-paraffin. One probable path is the skeletural isomerization of n-paraffin to branched paraffin and its cracking.
90 9Sel" of Isomerization
~.60
I
~
g 50 -
.~70 _ Sel. of isomerization _-.00_ 0 o ---~. > 60 "0 50 -Sel. of C r a c k i n g ~
~u
ok.,. 40
S
4O
90 80
I
o
-- 30 6o 20 10
--(D 30 O9 20
t.;onversion
10
0 --
!
0
0.5
l
I
I
.1 1.5 2 W/F(g-h/moi)
0
2.5
- ~ o n --
0
!
0.5
!
I
I
2 1 1.5 W/F(g.h/mol)
J
2.5
503 K, H2/C7=9, 1.0 MPa, Pd/SiO2:H-ZSM-5=I:I, W/F=I.2 g h mol-1 Figure 4. Hydrocracking of 2-methylhexane. Figure 5. Hydrocracking of n-heptane. Figures. 4 and 5 show the results of hydrocracking of 2-methylhexane (2MH) and nheptane (NHP), respectively. It is clear from the two figures that 1) NHP was more reactive than 2MH and that 2) while 2MH was first isomerized and then cracked, NHP was isomerized and cracked, paraUelly. These facts indicate that 2MH is first isomerized to NHP and then cracked to give propane and iso-butane. This apparently inconsistent phenomena can not be explained reasonably by the generally accepted classical theory of acid-catalyzed cracking, which assumes a B-scission mechanism for breakages of a C-C bond in a classical type of carbenium ion. It can be explained reasonably by the reaction path shown in Figure 6. The essential point of this reaction mechanism is that the cracking reaction proceed through a intermediate containing the protonated dialkylcyclopropane structure which has been proposed by Sie [7]. The smallest paraffin chain that can be cracked according to the protonated cyclopropane mechanism is nC7. The isobutylene, which is one of a pair of the primary cracked products, will be hydrogenated to isobutane in the presence of hydrogen and palladium catalyst. As will be discussed later, the hydride (H-so) as a counter anion of proton (H§ which is formed in hydrogen spillover process, stabilizes the propyl-carbenium ion to give propane. Thus oligomerization of the cracked fragments and consecutive cracking reaction is prevented in the H2-Pd-hybrid system.
1009
H
H
H
H I H I H. I H
!,-ct,,I/q\l/q\l
9
H--c
I H
H
I c
I C--H
I H
H
H
ll
I H
H
~ I H~C
c
I H
H
I H. I .H
/Cz~-,,,C~ I / C . ~ I
.I
N
N
,;',--C
H
H
/c,~ §
I--C~H
I H I
H
H
I/C,---~---- ~~ / . ~ H--C "',+4~ H .C I .I /L,,~ .I H H H H N
.C--H .I H
Hydride Shift Hydride Shift Scission
~C +./
I
-,
+
H--C
H
H
H. / - ~ / .H H I/c....~...C,, [,,~c\ I ",,+,,~ H "C, "" I
C~H
H
+H2/Pd
Figure 6. Reaction model of n-heptane hydrocracking on Pd/SiO2-H-ZSM-5.
3.1.
n-C7 cracking on Pd-based USY catalysts
Figures 7 to 9 show catalytic activity of USY hybrid catalyst as a function of reaction time, carbon-number distribution of products and the composition of C4 products. The results were very similar to those of the H-ZSM-5 containing catalysts system. However, the USY containing catalysts need higher reaction temperature than the H-ZSM-5 containing catalysts. As was the case of the cracking with ZSM-5 hybrid catalyst, the catalytic activity was the highest for the hybrid catalyst under hydrogen pressure. The characteristic feature of the product distribution is that the reaction products of H2-hybrid catalyst system are only isomerized heptane and propane and equimolar amount of isobutane (a little n-C4H10 was formed), whereas the products on H-ZSM-5 alone distributed from C3 to C9 and C4 products contained all kind of paraffins and olefins. The reactivities of branched C7 were different from those of the H-ZSM-5 containing catalysts system. Figures 10 to 12 show the results of hydrocracking of 2,4-dimethylpentane (2,4TMP), 2-methylhexane (2MH) and n-heptane (NHP), respectively. It is clear from the two figures that 1) 2,4TMP was more reactive than 2MH and NHP and that 2) while 2MH and NHP were first isomerized and then cracked, 2,4TMP was isomerized and cracked, paraUelly. These facts indicate that 2MH and NHP were first isomerized to 2,4TMP and then cracked to give propane and iso-butane.
1010
100 90 80 _ 70 60 50 40 30 20 10 _ 0
.. cO .=..
,.(1) t>O
o
0
60 Pd-Hybrid
50 ~An ~30 "~ 20 r,o
Pd/SiO 2
Pd-Hybrid
10
0.5 1 1.5 Time on Stream(h)
0[
2
1 2 3 4 5 6 7 8 9101112 Carbon Number
603 K, H2/n-CT=9, 1.0 MPa, Pd/SiO2:USY=I'I, W/F=I.2 g h mol-] Figure 7. Hydrocracking of n-heptane with USY catalyst.
Figure 8. C-number distribution. The same experiments as Fig.7 (TOS=I h).
Pd-Hybrid!I-I~[~ . . .~. ._. : ~ i ' - :~: ' _- " i! ; ] USY 10 20 30 40 50 60 70 80 90 100 Distribution (mol%)
0
Figure 9. Distribution of C4 hydrocarbons formed in the hydrocracking of n-CT. The same experiments as Figure 8.
90
90
8o 70 8 60
=~ 80 ~ 70 8 60
90
.~
"80 70
8
40 .
so
~ 4o
~
100
100
100
3o
9 2O
Sel.of Isom 9 d~,tjo n(%)~
10 0
i:.,o
~/~ 30
,
I
I
'
I
0
0.5
1
1.5
2
2.5
W/F(g. h - m o l "1)
30
20 10 ~ S S e ' 0 _L-in o
~ Cracldng(%)
20 10
I
f
' '
I
I
0.5
~
1.s
2
2.s
W/F(g" h- m o l "1)
0 o
o.s
1
1.s
2
2.s
W/F(.q. h - m o l "1)
603 K, H2/C7---9, 1.0 MPa, Pd/SiO2:USY=I'I, W/F=2.4 g h mol-1 Figure 10. Hydrocracking of 2,4-dimethylpentane.
Figure 11. Hydrocracking of 2-methylhexane.
Figure 12. Hydrocracking of n-heptane.
1011 It has been suggested that formation of multibranched isomers from the feed and cracking are consecutive reactions [6]. Cracking of a normal paraffin must thus proceed through the stage of formation of monobranched isomers, dibranched isomers and finally cracked product as in Figure 11, because the high energy barrier for B-scission of monobranched carbenium ion. The isobutylene, which is one of a pair of the primary cracked product, will be hydrogenated to isobutane in the presence of hydrogen and palladium catalyst. As will be discussed later, the hydride (H-so) as a counter anion of proton (H+so), which is formed in hydrogen spillover process, stabilizes the propyl-carbenium ion to give propane. Thus oligomerization of the cracked fragments and consecutive cracking reaction is prevented in the H2-Pd-hybrid system. C-C-C-C-C-C-C H-so
H so
-H2 C-C=C + C-C-C-C
..... C-C-C-C-C-C-C slow
c-r C
~
c-~-c -c-c Hso
~ c-q=c + ~-c-c slow
+
C
.
Hso
c_ =c § c_ _c
fast
c_
_c
c_ c § c_c_c
n'so
Figure 13. Hydrocracking model of n-hexane with Pd-USY-hybrid catalyst.
3.3. Effects of hydrogen spillover and reaction model Experimental results and discussion shown above suggest that not only cracking activity but also cracking pattern were affected by synergistic effect of hydrogen addition and supported metal catalyst. In the hydroisomerization of n-pentane over hybrid catalyst containing H-ZSM-5 and supported noble metal catalyst, it was proposed that a hydrogen molecule spills over on to zeolite surface as a proton and a hydride, where proton promotes the acid catalyzed reaction such as skeletal isomerization, on the other hand, hydride ion stabilizes intermediate carbenium ion to prevent oligomerization and cracking to improve selectivity for isomerization [2]. If the hydride ion is not reacted with carbenium ion, the carbenium ion will leave from the acid site as olefin while leaving proton on the acid site. The olefin will be polymerized to higher hydrocarbons and then be cracked on the acid site. Same phenomenon should occur in this system. Hydrogen gas is dissociated on the palladium on SiO2 and spills over onto the H-ZSM-5. Hydrogen transfer between the particles as in the case of the hybrid catalyst is a well-known phenomenon [4, 5]. The spillover hydrogen presumably exist on the zeolite stn'face as protons and hydride. The proton promotes cracking reaction even at low reaction temperature. The hydride generated simultaneously stabilizes intermediate carbenium ion to prevent oligomerization of the cracked fragments and consecutive cracking reaction and
1012 promote isomerization of alkane. The model of hydrogen spillover in the hybrid catalyst is shown in Figure 12. HE H
Figure 14. Hydrogen spillover model on Pd-hybrid catalyst.
4. CONCLUSION A hybrid catalyst, which was prepared by physical mixing of a H-ZSM-5 and Pd/SiO2, showed an excellent activity for the hydrocracking of n-paraffins. Hydrogen gas is dissociated on the palladium on SiO2 and spills over onto the H-ZSM-5. The spillover hydrogen presumably exist on the zeolite surface as protons and hydride. The proton promotes cracking reaction. The hydride generated simultaneously stabilizes intermediate carbenium ion to prevent over-cracking and promote isomerization of alkane. In the n-heptane cracking, the hybrid catalyst gave only isomerized heptane and propane and equimolar amount of i-butane whereas the products on H-ZSM-5 alone distributed from C3 to C9 and C4 products contained all kind of paraffins and olefins. The simple products for the H2-hybrid system should be formed through no other reaction path than the primary cracking reaction on H-ZSM-5, which proceeds through a intermediate containing the protonated dialkylcyclopropane structure.
REFERENCES
1. B.S. Greensfelder, H.H. Voge and G.M. Good, Ind. Eng. Chem., 41 (1949) 2573. 2. K. Fujimoto, K. Maeda and K. Aimoto, Applied Catal., 91 (1992) 81. 3. I. Nakamura, R. Iwamoto and A. I-ino, in "New Aspects of Spillover Effect in Catalysis" (T. Inui, K. Fujimoto, T. Uchijima and M. Masai, eds.), Elsevier, Amsterdam, 1993, pp. 77-84. 4. S.Khoobiar, J. Phys. Chem., 68 (1964) 411. 5. A.J. Robell, E.V. Ballou and M. Boudart, J. Phys. Chem., 68 (1964) 2748. 6. M. Steijns, G. Froment, P. Jacobs, J. Uytterhoeven and J. Weitkamp, Ind. Eng. Chem. Prod. Res. Dev., 20 (1981) 654. 7. S.T. Sie, Ind. Eng. Chem. Res. 31 (1992) 1881.
1005
Low temperature hydrocracking of paraffinic hydrocarbons over hybrid catalysts I. Nakamura, K. Sunada and K. Fujimoto Depamnent of Applied Chemistry, Faculty of Engineering, The University of Tokyo 7-3-1, Hongo, Bunkyo-ku, Tokyo 113 Japan
A hybrid catalyst, which was prepared by physical mixing of a H-ZSM-5 and Pd/SiO2. showed an excellent activity for the hydrocracking of n-paraffins at low reaction temperature (503 K). In the n-heptane cracking, the hybrid catalyst gave only isomerized heptane and propane and equimolar amount of i-butane whereas the products on H-ZSM-5 alone distributed from C3 to C9 and C4 products contained all kind of paraffins and olefins. The wide product distribution for H-ZSM-5 system should be attributed to the reaction path comprising oligomerization and cracking of the olig0mer. The simple products for the H2-hybrid system should be formed through no other reaction path than the primary cracking reaction on H-ZSM-5, which proceeds through a intermediate containing the protonated dialkylcyclopropane structure.
I. INTRODUCTION Hydrocracking of petroleum heavy hydrocarbons have been practiced extensively commercially in petroleum refining to produce high quality gasoline, jet fuel, gas oil and lubricants. Many hydrocracking catalysts of commercial importance are dual functional catalysts containing both hydrogenation components such as sulfided Ni-Mo or Ni-W and acidic components such as zeolites. The most predominant reaction mechanism for the hydrocracking of alkane is as follows: (1) the dehydrogenation of alkane to alkene on the supported metal; (2) proton addition to the alkene to form carbenium ion on the acidic component; (3) B-scission of the carbenium ion to form smaller carbenium ion and alkene on the acid component; (4) hydrogenation of the cracked alkene to alkane on the supported metal [1]. On the other hand, it was proposed that acid catalyzed reactions such as skeletal isomerization of paraffins [2] and disproportionation [3] over metal supported acid catalysts were promoted by spillover hydrogen (proton) on the acid catalysts. Hydrogen spillover phenomenon from noble metal to other component at room temperature has been reported in many cases [4, 5]. In the present work, hydrocracking at low reaction temperature was studied using hybrid catalysts containing H-ZSM-5 zeolite and a supported noble metal from the standpoint of hydrogen spillover.
1006
2. EXPERIMENTAL 2.1
Catalyst preparation Zeolites used were a H-ZSM-5 (Toso, HSZ-840NHA SIO2/A1203ratio=44.0) and USY zeolite (Catalyst & Chemicals Ind. SiO2/A1203 ratio=8.6 ). Silica-supported palladium was prepared by impregnating a commercially available SiO2 (Aerosil 380, BET specific surface area 380 m2/g) with PdC12 from its aqueous hydrochrolic solution which was followed by the calcination in air at 723 K for 3 h and the reduction in flowing hydrogen at 723 K for 1 h. Hybrid catalyst was prepared by co-grinding the mixture of 4 weight parts of the H-ZSM-5 with one weight part of Pd/SiO2 (2.5wt%) and pressure molding the mixture to granules to 20/40 mesh. Catalysts were activated in air at 723 K for 2 h and reduced in flowing hydrogen at 673 K for lh, before use. 2.2 Reaction apparatus and procedure The hydrocracking of n-paraffins was conducted with a continuous flow type fixed bed reaction apparatus under pressurized conditions. The reactor was a stainless steel tube with an inner diameter of 6 mm. The feed material which had been deeply desulfurized was fed by a liquid pump. The mole ratio of H2/n-CTH16 or N2/n-C7H16 in the feed was 9:1. Products were withdraw in the gaseous state and analyzed by a capillary gas chromatograph.
3. RESULTS AND DISCUSSION
3.1.
n-C7 cracking on Pd-based ZSM-5 catalysts Hydrocracking of n-heptane, which should give only C3 and C4 hydrocarbons in acid catalyzed cracking was studied. Figure 1 shows the changes of catalytic activities of a variety of catalysts containing Pd/SiO2 and/or H-ZSM-5 for n-heptane hydrocracking as a function of reaction time. The catalytic activity of H-ZSM-5 was not affected by the atmosphere and decreased quickly. Pd/SiO2 showed little activity for both dehydrogenation and cracking of n-CTH16. On the other hand, the catalytic activity of a hybrid catalyst comprising Pd/SiO2 and H-ZSM-5 was the highest and its activity was kept constant under hydrogen atmosphere while it was much lower and decreased quickly under nitrogen atmosphere. This phenomenon clearly shows that the presence of hydrogen is essential in order to generate hydrocracking activity. It is well known that the supported platinum shows a high catalytic activity for the dehydrogenation of alkane whereas the supported palladium does not. The results shown in Figure 1 suggest that the dehydrogenation activity of supported metal is not essential for the appearance of the alkane hydrocracking activity. The essential point is that the hydrogen-activating component intimately contacts with acidic catalyst. The present authors have pointed out that the skeletal isomerization of lower alkane is effectively promoted by the hybrid catalyst composed of Pd/SiO2 or Pt/SiO2 and H-ZSM-5 and that the hydrogen spillover is the key step of isomerization reaction. In the present case also, the hydrogen migration from Pd/SiO2 to H-ZSM-5 should be essential for the high and stable catalytic activity. The characteristic feature of the product distribution is that the reaction products of H2-hybrid catalyst system are only isomerized heptane and propane and equimolar amount of
1007 isobutane (little n=C4HI 0 was formed), whereas the products on H-ZSM,5 alone distributed from C3 to C9 and C4 products contained all kind of paraffins and olefins as shown in Figure 2. The wide product distribution for H-ZSM-5 system should be attributed to the reaction path comprising polymerization and cracking. In the hydroisomerization of n-pentane over hybrid catalyst containing H--ZSM--5 and supported noble metal catalyst, it was proposed that a hydrogen molecule spills over on to zeolite surface as a proton and a hydride [2]. The proton promotes cracking reaction even at low reaction temperature. The hydride generated simultaneously stabilizes intermediate carbenium ion to prevent oligomerization and cracking of oligomer and promote isomerization of alkane. The simple products for the H2-hybrid system should be formed through no other reaction path than the primary cracking reaction on H-ZSM-5. 70 ~ 60 -
60
Hybrid(H2)
[[I •
Hybrid(H 2)
50-H.ZSM.5(H2 )
~50 -
~4o
c
o 40 -
.m
30 r-o ~~
-
H-ZSM-5(H 2)
~2o
O 20 -
10
I
10 Pd!Si02(H2) . . . . -
0
-
I
o~
I
0.5 1 1.5 2 Time on Stream(h)
2.5
~
1 2 3 4 5 6 7 8 9 1011 12 Carbon Number
503 K, H2 or N2/n-C7--9, 1.0 MPa, Pd/SiO2:H-ZSM-5=I:I, W/F=I.2 g h mol-1 Figure 1. Hydrocracking of n-heptane with ZSM-5 catalysts.
Figure 2. C-number distribution. The same experiments as Figure 1 (TOS=I h).
Hybrid I-I2 ........................................................... Hybrid N2 ~....:~....: 4 ~ i ]
C~
n-C4
|
H-ZSM-5 H lltl~Qtm~l~R..:.~ H-ZSM-5 N2 ~
~
i
i
I
I
I
I
I
I
I
I
0
10
20
30
40
50
60
70
80
I 90 100
Distribution (mol%) Figure 3. Distribution of C4 hydrocarbons formed in the hydrocracking of n-C7. The same experiments as Figure 2.
1008 In hydrocracking of normal paraffin with metal supported acid catalyst, the iso/normal ratios in the paraffinic products generally exceed the thermodynamic equilibrium. It proves that at least some of the branched paraffins are primary products of the cracking and not a results of the post isomerization. This is particularly true in the case of C4, since n-butane cannot be isomerized under typical hydrocracking conditions with a zeolite catalyst. Especially the fact that isobutane is the sole C4 product in the present system as shown in Figure 3 suggests that the hydrocracking on the hybrid catalyst should not proceed through the cracking of n-paraffin to form smaller n-paraffin and another n-paraffin. One probable path is the skeletural isomerization of n-paraffin to branched paraffin and its cracking.
90 9Sel" of Isomerization
~.60
I
~
g 50 -
.~70 _ Sel. of isomerization _-.00_ 0 o ---~. > 60 "0 50 -Sel. of C r a c k i n g ~
~u
ok.,. 40
S
4O
90 80
I
o
-- 30 6o 20 10
--(D 30 O9 20
t.;onversion
10
0 --
!
0
0.5
l
I
I
.1 1.5 2 W/F(g-h/moi)
0
2.5
- ~ o n --
0
!
0.5
!
I
I
2 1 1.5 W/F(g.h/mol)
J
2.5
503 K, H2/C7=9, 1.0 MPa, Pd/SiO2:H-ZSM-5=I:I, W/F=I.2 g h mol-1 Figure 4. Hydrocracking of 2-methylhexane. Figure 5. Hydrocracking of n-heptane. Figures. 4 and 5 show the results of hydrocracking of 2-methylhexane (2MH) and nheptane (NHP), respectively. It is clear from the two figures that 1) NHP was more reactive than 2MH and that 2) while 2MH was first isomerized and then cracked, NHP was isomerized and cracked, paraUelly. These facts indicate that 2MH is first isomerized to NHP and then cracked to give propane and iso-butane. This apparently inconsistent phenomena can not be explained reasonably by the generally accepted classical theory of acid-catalyzed cracking, which assumes a B-scission mechanism for breakages of a C-C bond in a classical type of carbenium ion. It can be explained reasonably by the reaction path shown in Figure 6. The essential point of this reaction mechanism is that the cracking reaction proceed through a intermediate containing the protonated dialkylcyclopropane structure which has been proposed by Sie [7]. The smallest paraffin chain that can be cracked according to the protonated cyclopropane mechanism is nC7. The isobutylene, which is one of a pair of the primary cracked products, will be hydrogenated to isobutane in the presence of hydrogen and palladium catalyst. As will be discussed later, the hydride (H-so) as a counter anion of proton (H§ which is formed in hydrogen spillover process, stabilizes the propyl-carbenium ion to give propane. Thus oligomerization of the cracked fragments and consecutive cracking reaction is prevented in the H2-Pd-hybrid system.
1009
H
H
H
H I H I H. I H
!,-ct,,I/q\l/q\l
9
H--c
I H
H
I c
I C--H
I H
H
H
ll
I H
H
~ I H~C
c
I H
H
I H. I .H
/Cz~-,,,C~ I / C . ~ I
.I
N
N
,;',--C
H
H
/c,~ §
I--C~H
I H I
H
H
I/C,---~---- ~~ / . ~ H--C "',+4~ H .C I .I /L,,~ .I H H H H N
.C--H .I H
Hydride Shift Hydride Shift Scission
~C +./
I
-,
+
H--C
H
H
H. / - ~ / .H H I/c....~...C,, [,,~c\ I ",,+,,~ H "C, "" I
C~H
H
+H2/Pd
Figure 6. Reaction model of n-heptane hydrocracking on Pd/SiO2-H-ZSM-5.
3.1.
n-C7 cracking on Pd-based USY catalysts
Figures 7 to 9 show catalytic activity of USY hybrid catalyst as a function of reaction time, carbon-number distribution of products and the composition of C4 products. The results were very similar to those of the H-ZSM-5 containing catalysts system. However, the USY containing catalysts need higher reaction temperature than the H-ZSM-5 containing catalysts. As was the case of the cracking with ZSM-5 hybrid catalyst, the catalytic activity was the highest for the hybrid catalyst under hydrogen pressure. The characteristic feature of the product distribution is that the reaction products of H2-hybrid catalyst system are only isomerized heptane and propane and equimolar amount of isobutane (a little n-C4H10 was formed), whereas the products on H-ZSM-5 alone distributed from C3 to C9 and C4 products contained all kind of paraffins and olefins. The reactivities of branched C7 were different from those of the H-ZSM-5 containing catalysts system. Figures 10 to 12 show the results of hydrocracking of 2,4-dimethylpentane (2,4TMP), 2-methylhexane (2MH) and n-heptane (NHP), respectively. It is clear from the two figures that 1) 2,4TMP was more reactive than 2MH and NHP and that 2) while 2MH and NHP were first isomerized and then cracked, 2,4TMP was isomerized and cracked, paraUelly. These facts indicate that 2MH and NHP were first isomerized to 2,4TMP and then cracked to give propane and iso-butane.
1010
100 90 80 _ 70 60 50 40 30 20 10 _ 0
.. cO .=..
,.(1) t>O
o
0
60 Pd-Hybrid
50 ~An ~30 "~ 20 r,o
Pd/SiO 2
Pd-Hybrid
10
0.5 1 1.5 Time on Stream(h)
0[
2
1 2 3 4 5 6 7 8 9101112 Carbon Number
603 K, H2/n-CT=9, 1.0 MPa, Pd/SiO2:USY=I'I, W/F=I.2 g h mol-] Figure 7. Hydrocracking of n-heptane with USY catalyst.
Figure 8. C-number distribution. The same experiments as Fig.7 (TOS=I h).
Pd-Hybrid!I-I~[~ . . .~. ._. : ~ i ' - :~: ' _- " i! ; ] USY 10 20 30 40 50 60 70 80 90 100 Distribution (mol%)
0
Figure 9. Distribution of C4 hydrocarbons formed in the hydrocracking of n-CT. The same experiments as Figure 8.
90
90
8o 70 8 60
=~ 80 ~ 70 8 60
90
.~
"80 70
8
40 .
so
~ 4o
~
100
100
100
3o
9 2O
Sel.of Isom 9 d~,tjo n(%)~
10 0
i:.,o
~/~ 30
,
I
I
'
I
0
0.5
1
1.5
2
2.5
W/F(g. h - m o l "1)
30
20 10 ~ S S e ' 0 _L-in o
~ Cracldng(%)
20 10
I
f
' '
I
I
0.5
~
1.s
2
2.s
W/F(g" h- m o l "1)
0 o
o.s
1
1.s
2
2.s
W/F(.q. h - m o l "1)
603 K, H2/C7---9, 1.0 MPa, Pd/SiO2:USY=I'I, W/F=2.4 g h mol-1 Figure 10. Hydrocracking of 2,4-dimethylpentane.
Figure 11. Hydrocracking of 2-methylhexane.
Figure 12. Hydrocracking of n-heptane.
1011 It has been suggested that formation of multibranched isomers from the feed and cracking are consecutive reactions [6]. Cracking of a normal paraffin must thus proceed through the stage of formation of monobranched isomers, dibranched isomers and finally cracked product as in Figure 11, because the high energy barrier for B-scission of monobranched carbenium ion. The isobutylene, which is one of a pair of the primary cracked product, will be hydrogenated to isobutane in the presence of hydrogen and palladium catalyst. As will be discussed later, the hydride (H-so) as a counter anion of proton (H+so), which is formed in hydrogen spillover process, stabilizes the propyl-carbenium ion to give propane. Thus oligomerization of the cracked fragments and consecutive cracking reaction is prevented in the H2-Pd-hybrid system. C-C-C-C-C-C-C H-so
H so
-H2 C-C=C + C-C-C-C
..... C-C-C-C-C-C-C slow
c-r C
~
c-~-c -c-c Hso
~ c-q=c + ~-c-c slow
+
C
.
Hso
c_ =c § c_ _c
fast
c_
_c
c_ c § c_c_c
n'so
Figure 13. Hydrocracking model of n-hexane with Pd-USY-hybrid catalyst.
3.3. Effects of hydrogen spillover and reaction model Experimental results and discussion shown above suggest that not only cracking activity but also cracking pattern were affected by synergistic effect of hydrogen addition and supported metal catalyst. In the hydroisomerization of n-pentane over hybrid catalyst containing H-ZSM-5 and supported noble metal catalyst, it was proposed that a hydrogen molecule spills over on to zeolite surface as a proton and a hydride, where proton promotes the acid catalyzed reaction such as skeletal isomerization, on the other hand, hydride ion stabilizes intermediate carbenium ion to prevent oligomerization and cracking to improve selectivity for isomerization [2]. If the hydride ion is not reacted with carbenium ion, the carbenium ion will leave from the acid site as olefin while leaving proton on the acid site. The olefin will be polymerized to higher hydrocarbons and then be cracked on the acid site. Same phenomenon should occur in this system. Hydrogen gas is dissociated on the palladium on SiO2 and spills over onto the H-ZSM-5. Hydrogen transfer between the particles as in the case of the hybrid catalyst is a well-known phenomenon [4, 5]. The spillover hydrogen presumably exist on the zeolite stn'face as protons and hydride. The proton promotes cracking reaction even at low reaction temperature. The hydride generated simultaneously stabilizes intermediate carbenium ion to prevent oligomerization of the cracked fragments and consecutive cracking reaction and
1012 promote isomerization of alkane. The model of hydrogen spillover in the hybrid catalyst is shown in Figure 12. HE H
Figure 14. Hydrogen spillover model on Pd-hybrid catalyst.
4. CONCLUSION A hybrid catalyst, which was prepared by physical mixing of a H-ZSM-5 and Pd/SiO2, showed an excellent activity for the hydrocracking of n-paraffins. Hydrogen gas is dissociated on the palladium on SiO2 and spills over onto the H-ZSM-5. The spillover hydrogen presumably exist on the zeolite surface as protons and hydride. The proton promotes cracking reaction. The hydride generated simultaneously stabilizes intermediate carbenium ion to prevent over-cracking and promote isomerization of alkane. In the n-heptane cracking, the hybrid catalyst gave only isomerized heptane and propane and equimolar amount of i-butane whereas the products on H-ZSM-5 alone distributed from C3 to C9 and C4 products contained all kind of paraffins and olefins. The simple products for the H2-hybrid system should be formed through no other reaction path than the primary cracking reaction on H-ZSM-5, which proceeds through a intermediate containing the protonated dialkylcyclopropane structure.
REFERENCES
1. B.S. Greensfelder, H.H. Voge and G.M. Good, Ind. Eng. Chem., 41 (1949) 2573. 2. K. Fujimoto, K. Maeda and K. Aimoto, Applied Catal., 91 (1992) 81. 3. I. Nakamura, R. Iwamoto and A. I-ino, in "New Aspects of Spillover Effect in Catalysis" (T. Inui, K. Fujimoto, T. Uchijima and M. Masai, eds.), Elsevier, Amsterdam, 1993, pp. 77-84. 4. S.Khoobiar, J. Phys. Chem., 68 (1964) 411. 5. A.J. Robell, E.V. Ballou and M. Boudart, J. Phys. Chem., 68 (1964) 2748. 6. M. Steijns, G. Froment, P. Jacobs, J. Uytterhoeven and J. Weitkamp, Ind. Eng. Chem. Prod. Res. Dev., 20 (1981) 654. 7. S.T. Sie, Ind. Eng. Chem. Res. 31 (1992) 1881.