149
CHAPTER 5
1-OCTENE ISOMERIZATION
5.1
INTRODUCTION
1-octene isomerization was undertaken with a view to determine
whether the trend observed in 1-hexene isomerization on all the catalysts
during the change of reaction parameter are also observed in 1-octene
isomerization or not, to study the difference in the nature and amount of
coke deposits formed and to ascertain whether the acidity order of catalysts
determined from total skeletal isomerization products in hexene
isomerization are also observed in this reaction or not. The following
sections discuss the effects of various parameters on the 1-octene conversion,
product yield and their selectivity.
5.2
PRODUCT DISTRIBUTION
When 1-octene is passed over the acidic catalysts it undergoes double
bond isomerization to yield 2-octene, 3-octene and 4-octene. The product
octenes can undergo skeletal isomerization to yield methylheptenes and
dimethylhexenes. Both reactant and products can be cracked to yield C3-C6
olefins and paraffins.
Double bond isomerization
1 - octene
> 2 - octene, 3 - octene and 4 - octene
150
Skeletal isomerization
2 - octene
3 - octene
----- >
4 - octene
Methylheptenes and
dimethylhexenes
Cracking
Propylene, propane, n-butane,
isobutane, butenes, isobutene,
Octenes ------ >
methylbutanes, methylbutenes, pentenes,
methylpentenes and hexane.
5.3
EFFECTS OF REACTION TEMPERATURE
The catalysts with same SiO^BgOg ratios and similar experimental
conditions used in hexene isomerization were used in this 1- octene
isomerization.
Isomerization of 1-octene was studied in the temperature range
473-623 K on the H-form of A(20), A(50), A(100), A(200) and Al-ZSM - 5(20)
catalysts all of which were activated at 773 K. The GC analysis of the
products mixture shows the presence of 2-octene, 3-octene, 4-octene,
methylheptenes and dimethylhexenes. A typical chromatogram is shown in
Figure 5.1. The distribution of the product obtained are given in Table 5.1,
5.2, 5.3, 5.4 and 5.5. The results of same reaction over B(20), B(50), B(100),
B(200) and A1-MCM-41(20) catalysts under similar experimental conditions
are given in Tables 5.6, 5.7, 5.8, 5.9 and 5.10 respectively. In this
temperature range, the double bond shift is dominant, forming 2-octene,
3-octene and 4-octene. Skeletal isomerization products such as
methylheptenes and dimethylhexenes are found to form at and above 573
K with both A and B series of boron catalysts. The skeletal isomerization
Intensity (arb- units)
Figure 5.1
Typical gas chromatogram of the reaction produc i
the 1-octene isomerisation
152
Table 5.1
Effect of reaction temperature on product distribution of 1-octene isomerisation
Catalyse A (20)
Product
1-Octene
WHSV=3 h'1
Time on stream=l h
Product yield ( wt. %) at different
temperature
523 K
473 K
573 K
623 K
78.5
45.0
16.4
8.7
Reaction
type3
-
2-Octene
7.9
14.0
10.4
7.8
DBS
3-Octene
10.7
24.9
20.9
16.4
DBS
4-Octene
3.9
14.3
22.3
19.2
DBS
Methylheptenes
-
1.5
20.3
29.2
SR
Dimethylhexenes
-
-
9.2
18.7
SR
a DBS-Double Bone Shift ; SR- Skeletal Rearrangement
Table 5.2
Effect of reaction temperature on product distribution of 1-octene isomerisation
Catalyst= A (50)
Product
WHSV=3 h'1
Time on stream=l h
Product yield ( wt. %) at different
temperature
523 K
573 K
623 K
473 K
Reaction
type3
1-Octene
84.0
50.0
20.1
8.9
-
2-Octene
5.6
15.5
12.4
10.9
DBS
3-Octene
8.2
24.5
23.3
19.1
DBS
4-Octene
2.2
10.0
23.8
20.2
DBS
Methylheptenes
-
-
15.5
26.4
SR
Dimethylhexene
-
-
4.9
14.4
SR
a DBS-Double Bond Shift; SR- Skeletal Rearrangement
153
Table 5.3
Effect of reaction temperature on product distribution of 1-octene isomerisation
Catalyst= A (100)
Product
WHSV=3 h'1
Time on slream=l h
Product yield ( wt.%) at different
temperature
573 K
623 K
523 K
473 K
Reaction
type3
1 -Oclene
85.5
54.4
26.6
8.5
-
2-Octene
5.6
16.8
15.4
14.0
DBS
3-Oclene
7.8
24.2
22.8
22.4
DBS
4-Octene
1.0
4.0
21.4
22.6
DBS
MethyUieptenes
-
-
10.3
22.7
SR
Dimethylhexene
-
-
3.5
9.7
SR
a DBS-Double Bond Shift; SR- Skeletal Rearrangement
Table 5.4
Effect of reaction temperature on product distribution of 1-octene isomerisation
Catalyst=A (200)
Product
1-Octene
WHSV=3 h1
Time on stream=l h
Product yield ( wt. %) at different
temperature
523 K
573 K
623 K
473 K
89.0
57.0
35.7
13.5
Reaction
type3
-
2-Octene
4.5
17.2
16.8
16.5
DBS
3-Octene
5.9
22.7
22.4
21.9
DBS
4-Octene
0.6
3.1
19.8
21.8
DBS
MethyUieptenes
-
-
4.3
19.2
SR
7.1
SR
Dimelhylhexenes
1.0
a DBS-Double Bond Shift; SR- Skeletal Rearrangement
-
-
154
Table 5.5
Effect of reaction temperature on product distribution of 1-octene isomerisation
Catalyst=Al-ZSM-5 (20)
WHSV=3 h"1
Time on stream^! h
Product yield ( wt. %) at different
temperature
623 K
473 K
523 K
573 K
7.5
71.0
39.5
9.0
Product
1-Octene
Reaction
type
-
2-Octene
8.1
11.5
9.1
6.0
DBS
3-Octene
14.8
21.2
19.1
13.9
DBS
4-Oelene
6.1
14.5
23.9
18.5
DBS
Mcthylheptenes
-
9.4
24.3
29.1
SR
Dimethylhexenes
-
3.9
14.5
18.5
SR
Others
6.5
a DBS-Double Bond Shift; SR- Skeletal Rearrangement; C-Cracking.
-
-
-
C
Table 5.6
Effect of reaction temperature on product distribution of 1-octene isomerisation
Catalyst= B (20)
WHSV=3 If1
Time on stream=l h
Product yield ( wt. %) at different temperature
Product
Reaction
type8
473 K
523 K
573 K
623 K
1 -Octene
85.0
46.2
30.3
25.6
-
2-Octene
5.2
18.0
16.0
12.5
DBS
3-Octene
8.1
28.5
27.1
22.9
DBS
4-Octene
1.7
7.3
19.0
24.0
DBS
-
6.0
10.0
SR
-
1.6
5.0
SR
Mcthylheptenes
Dimethylhexenes
-
a DBS-Double Bond Shift; SR- Skeletal Rearrangement
155
Table 5.7
Effect of reaction temperature on product distribution of 1-octene isomerisation
Catalyst=B (50)
Product
WHSV=3 h'1
Time on stream=l h
Product yield ( wt. %) at different
temperature
623 K
573 K
523 K
473 K
Reaction
type
1 -Octene
88.5
53.0
40.0
37.1
-
2-()ctene
4.4
16.8
14.5
11.6
DBS
3-Octene
6.4
25.9
23.9
20.5
DBS
4-Octene
Methylheptene
0.7
4.3
16.0
19.0
DBS
-
-
4.2
8.0
SR
1.4
2.9
DBS-Double Bond Shift; SR- Skeletal Rearrangement
SR
Dimethylliexen
-
-
Table 5.8
Effect of reaction temperature on product distribution of 1-octene isomerisation
Catalyst=B (100)
Product
WHSV=3 h1
Time on stream=l h
Product yield ( wt. %) at different
temperature
523 K
573 K
623 K
473 K
Reaction
type8
1-Octene
92.0
60.6
48.6
42.7
2-Octene
3.1
15.0
13.5
12.0
DBS
3-Octene
4.5
22.0
21.5
20.3
DBS
4-Octene
0.4
24.0
13.2
17.9
DBS
Methylheptenes
-
-
3.2
6.0
SR
1.1
SR
Dimethylhexenes
a DBS-Double Bond Shift: SR- Skeletal Rearrangement
-
-
-
-
156
Table 5.9
Effect of reaction temperature on product distribution of 1-octene isomerisation
Catalyst=B (200)
Product
WHSV=3 h 1
Time on stream=l h
Product yield ( wt. %) at different
temperature
473 K
523 K
623 K
573 K
Reaction
type3
1-Octene
93.5
75.9
59.7
45.9
-
2-Octene
2.7
9.7
12.0
14.9
DBS
3-Octene
3.6
13.4
19.3
24.3
DBS
4-Octene
0.2
1.0
6.0
10.9
DBS
-
-
2.0
4.0
SR
-
-
-
-
SR
Methylheptenes
Dimethylliexen
a DBS-Double Bond Shift; SR- Skeletal Rearrangement
Table 5.10
Effect of reaction temperature on product distribution of 1-octene isomerisation
Catalyst= A1-MCM-41(20)
Product
1-Octene
WHSV=3 h1
Time on stream=l h
Product yield ( wt. %) at different
temperature
473 K
523 K
573 K
623 K
42.0
74.0
14.0
10.0
Reaction
type3
-
2-Octene
8.2
12.2
11.3
7.2
DBS
3-Octene
13.0
20.3
19.6
14.7
DBS
4-Octene
4.8
13.6
21.8
13.6
DBS
Methylheptenes
-
8.5
21.0
29.9
SR
Dimethylhexen
-
3.4
12.3
20.8
SR
Others
-
-
-
3.8
C
a DBS-Double Bond Shift; SR- Skeletal Rearrangement; C-Cracking.
157
products are found to form at a much higher temperature with boron
substituted catalysts compared to the aluminium substituted catalysts.
Also the amount of skeletal isomerization products formed over B-ZSM-5
and B-MCM-41 catalysts are less compared with that over corresponding
Al-ZSM-5 and Al-MCM-41. This is attributed to the decrease of strength of
Brpnsted acid sites of catalysts as a result of substitution of boron for
aluminium in the framework of catalysts.
The effects of temperature on conversion of 1-oetene on A and B series
catalysts along with aluminium substituted catalysts at WHSV 3 h'1 are
presented in Figure 5.2 and 5.3 respectively. The conversion of 1-octene
increases with an increase of temperature in all the catalysts. At all
temperatures higher conversions were obtained with aluminium substituted
catalysts than boron substituted catalysts in both sets of catalyst systems.
At a given constant temperature, the decrease of boron content in the
framework of both series catalysts (increase of Si02/B203 ratio) reduces the
conversion of 1-octene. For example, using catalysts A(20), A(50), A(100) and
A(200) 83.6, 80, 73.4 and 64.3% conversion respectively were obtained. The
decrease of conversion is attributed due to the decrease of number of acid
sites present in the catalyst, as Si02/B203 ratio increases.
The yield of each of 2-octene, 3-octene, and 4-octene is found increase
with an increase of temperature from 473 to 523 K in all the catalysts. With
further increase of temperature to 623 K through 573 K, the yields of
2-octene and 3-octene decrease and that of 4-octene, methylheptenes and
dimethylhexenes increase with all the catalysts of A series and B(20), B(50)
and B(100) of B series. The decrease of 2-octene and 3-octene yields with an
increase of temperature above 523 K could be due to the conversion of
2-octene and 3-octene formed into 4-octene, methylheptenes and
dimethylhexenes. With B(200) the yield of 2-octene, 3-octene and 4-octene
increases with an increase of temperature from 473 to 623 K. This is due to
reduced rate of transformation of double bond shift products to skeletal
Conversion (wt%)
158
-O-A(20)
—■— A<50)
—A—A(100)
—O—
473
523
A(200)
Ai-ZSM-5(20)
573
623
Temperature (K)
Figure 5.2 Effect of temperature on conversion of 1-0ctene
over A series and AI-ZSM-5(20)
WHSV=3 h 1, Time on stream=1 h
Figure 5.3 Effect of temperature on conversion of 1-0ctene
over B series and AI-MCM-41(20)
WHSV=3 h 1, Time on stream=1 h
159
isomerization products. Among the boron substituted catalysts studied,
B(200) possesses a low number of acidic sites and reduces the rate of
transformation of double bond shift products to skeletal isomerization.
The mechanism of formation of products is supposed to involve
interaction of 1-octene with the acid sites of catalysts leading to the
formation of C8 carbenium ion (I). The carbenium ion can undergo
successive hydrogen shift to form Cg linear carbenium ions (II) and (III).
The formation of 2-octene is attributed to deprotonation of carbenium ions
(I) and (II). Similarly 3-octene formation is attributed to deprotonation of
carbenium ions (II) and (III) and that of 4-octene to the deprotonation of
carbenium ion (III). Methylheptenes and dimethyl hexenes are formed via
deprotonation of branched carbenium ions (IVa-IVe) and (V) respectively.
Carbenium ions (IVa-IVe) and (V) inturn are formed by a sequence of
hydrogen shifts and methylshifts from carbenium ions (I) - (III) (Scheme
5.1).
Bar diagrams 5.4 to 5.8 show product selectivity of 1-octene
isomerization at different temperatures of A(20), A(50), A(100), A(200)
and Al-ZSM-5(20). The product selectivity in the same reaction under
similar experimental conditions of the corresponding B series catalysts and
A1-MCM-41(20) are shown in diagrams 5.9 to 5.13. It is observed that with
all the catalysts an increase of temperature from 473 to 623 K, decreases
the selectivity of 2-octene and 3-octene with simultaneous increase in
selectivity of methylheptenes and dimethylhexenes. This change in
selectivity is very much obvious in the temperature range 523-573 K.
The selectivity of 4-octene over catalysts Al-ZSM-5(20), AI-MCM-41(20),
A(20), A(50), A( 100) and A(200)is found to increase when temperature raises
from 473 K to 573 K. Further increase of temperature to 623 K, results in
small decrease in selectivity of 4-octene. But with B(20), B(50), B(100) and
B(200) selectivity of 4-octene increase steadily when temperature increases
from 473 to 623 K. The decrease in the selectivity of 2-octene and 3-octene
- i1
—
c ---- C — C — C ------ C
\
+
! ,2
methyl
IV
(e)
Scheme 5.1 Reaction network for isomerization of 1-octene
4-Octene
111
c — c — c — c— c — c — c — c
\
3-0ctene
+
C ----- C ----- C----- C ------C — C ----- C
/
C----
2-OcteW
c — c — c — c— c — c — c — c
+
2
Cat
-Octene
/
1
160
f=q
02-0
03-0
□ Mil
ODMH
04-0
ro
Selectivity (wt%)
161
473
523
573
623
Temperature (K)
Effect of temperature on product selectivity in
1 -octene isomerisation over catalyst A{20)
WHSV=3 h'1. Time on stream=1 h
03-0
□MH
mDMH
Q4-0
o
-£■
02-0
o
ro
Selectivity (wt%)
Figure 5.4
473
523
573
623
Temperature (K)
Figure 5.5
Effect of temperature on product selectivity in
1-octene isomerisation over catalyst A(50)
WHSV=3 h'1, Time on stream=1 h
162
Temperature (K)
Effect of temperature on product selectivity in
1-octene isomerisation over catalyst A(100)
WHSV=3 h‘\ Time on stream=1 h
E2-0
@3-0
□ MH
IlDMH
04-0
o
CM
0)
ro
OJ
cn
CO
ro
cn
CO
CO
mmmmM
Selectivity (wt%)
Figure 5.6
Temperature (K)
Figure 5.7
Effect of temperature on product selectivity in
1-octene isomerisation over catalyst A(200)
WHSV=3 h'\ Time on stream=1 h
60
02-0
@3-0
□ MH
IfflDMH
04-0
O
<%1
m
)
mm®
mmmmm
c\i
O
AjiAijoeies
0 >473
523
573
623
Temperature (K)
Figure 5.8
Effect of temperature on product selectivity in
1-octene isomerisation over catalyst AI-ZSM-5{20)
WHSV=3 h-1, Time on stream=1 h
02-0
@3-0
□ MH
BDMH
04-0
mmmm
CvJ
o
(%}M) AiiAipeies
Q
^
Temperature (K)
Figure 5.9
Effect of temperature on product selectivity in
1-octene isomerisation over catalyst B(20)
WHSV=3 h'1, Time on stream=1 h
164
02-0
03-0
□ 4-0
□ MH
^
ro
Selectivity (wt%)
ilDMH
473
523
573
623
Temperature (K)
Figure 5.10
Effect of temperature on product selectivity in
1-octene isomerisation over catalyst B(50)
WHSV=3 h'1, Time on stream=1 h
-p
^
a 3-0
□ MH
ru
Selectivity (wt%)
02-0
□ 4-0
11DMH
CO
_ •&
523
573
623
Temperature (K)
Figure 5.11
Effect of temperature on product selectivity in
1-octene isomerisation over catalyst B(100)
WHSV=3 h'1, Time on stream=1 h
Selectivity (wt%)
165
473
523
573
623
<g
Temperature (K)
re 5.12
Effect of temperature on product selectivity in
1-octene isomerisation over catalyst B(200)
WHSV=3 h'\ Time on stream=1 h
02-0 @3-0
□ 4-0 □ MH
tBDMH
40
Wmm.
Selectivity (wt%)
60
473
523
573
623
Temperature (K)
Figure 5.13 Effect of temperature on product selectivity in
1-octene isomerisation over catalyst
AI-MCM-41(20); WHSV=3 h'\ TOS=1 h
166
are
due
to
their
conversion
into
4-octene,
methylheptenes
and
dimethylhexenes. The decrease of 4-octene selectivity beyond 573 K on A
series boron catalysts and aluminium substituted catalysts is due to
conversion of more amount of 4-octene formed into methylheptenes and
dimethylhexenes as a result of greater acidic strength on the surface of
B-ZSM-5 catalysts such as A(20), A(50), A(100) and A(200) compared with
B-MCM-41 catalysts (Trong On et al 1995). The formation of 4-octene is
more
compared
to
its
transformation
into
methylheptene
and
dimethylhexenes with B-MCM-41 catalysts due to their low acidic strength.
The latter reaction does need stronger acidity, hence 4-octene selectivity
increases with B series catalysts beyond 573 K unlike that of A series
catalysts.
At all the reaction temperatures, a much higher selectivity of 2-octene
and 3-octene and considerably lower selectivity of 4-octene, methylheptenes
and dimethylhexenes are observed with B series catalysts compared with A
series. This is probably due to the combined effect of pore size and acidity
of the catalysts. Both sets of catalysts differ very much on these two
parameters. Skeletal isomerization products are formed by a sequence of
hydrogen and methylshifts. These are high energy demanding steps, and
hence need strong acidity. From the total skeletal isomerization products
formed at 623 K, the following acidity orders are observed among the
catalysts : Al-ZSM-5 > Al-MCM-41 = A(20) > A(50) > A(100) > A(200) >
B(20) > B(50) > B(100) > B(200). This order of arrangement of catalysts
based on the decrease in yield of skeletal isomerization products is exactly
similar to what was observed in 1-hexene isomerization over the same sets
of catalysts under similar experimental conditions.
Ratios of selectivity of double bond shift products [2- Octene/3-Octene
and 3-Octene/4-Octene] of 1-oetene isomerization at different temperatures
for all the catalysts are shown in Table 5.11. The Table shows that as the
temperature increases both ratios of 2-octene/3-oetene and 3-octene/4-octene
A(20)
A(50)
A(100)
Catalyst
0.73
0.63
B(200)
A1-MCM-41(20)
0.55
0.65
0.68
!
j
Al-ZSM-5(20)
B(20)
B(50)
o
o
<N
<
o
eo'
(ooi)a
473 K
0.64
0.69
0.72
0.76
o
0.59
0.60
0.64
0.67
0.57
oc
|
0.51
0.59
0.67
0.74
573 K
623 K
0.47
0.54
0.74
0.43
0.54
VO
VT,
OV
IT)
O d
18.6
2.7
!
1
vq
3.2
0.90
9.3
13.5
1.36
\6
i
1
1.0
0.75
623 K
0.85
0.95
0.99
2.2
(N
0.61
0.49
3.9
0.8
____________
IT,
14.1
2.4
4.9
1.13
____ U____
10.8
i
573 K
0.92
0.97
523 K
1.74
2.45
6.0
473 K
2.8
3.6
3-octene/4-octene ratio at different
temperature
Time on stream=l h
IT,
!
1
523 K
0.57
0.63
0.71
0.76
0.54
0.63
0.65
0.68
0.72
0.60
2-octene/3-octene ratio at different
temperature
o
vq
o'
l"6
1
90'1
WHSV=3 h
LL
Effect of temperature on ratios of selectivity of 1-octene double bond shift products
over A and B series catalysts
on
Table 5.11
167
168
decrease in all the catalysts. This indicates that a hydrogen shift in octyl
carbenium ions (I and II) increases compared to deprotonation as the
temperature increases. Similarly both ratios decrease with an increase of
boron content of catalysts (decrease of SiO^BgOg ratio) at all temperatures.
This is due to an increase of acidity in the catalyst with an increase of boron
content in the framework, which in turn enhances the hydrogen shift in
octyl carbenium ions. These observations are similar to what was observed
in hexene isomerization.
5.4
COKE FORMATION
The effect of temperature on the wt% coke formation in all the
catalysts in 1-octene isomerization at temperatures 473, 523, 573 and 623
K was studied at the end of 5th hour of the catalytic run and the results are
given in Table 5.12. TGA study of the coke deposited on catalysts shows the
presence of soft coke molecules (weight loss occurring below 773 K) in all the
boron substituted catalysts [A and B series] and Al-MCM-41 catalyst, in the
above mentioned temperature range. But in Al-ZSM-5 (20) coke molecules
formed in the reaction temperature range 473-573 K, are found to be soft
and that formed after 573 K but upto 623 K are hard black colourd coke
molecules (weight loss occur above 773 K) along with some soft coke
molecules. FTIR spectra of coke deposited catalysts in the above mentioned
temperature range shows the prominent IR band in the CH deformation
region around 1600 cm'1. A typical IR spectra is shown in Figure 5.14. It is
observed from IR spectra that, coke consists mainly of adsorbed olefins in
the octene isomerization like that in hexene isomerization in the
temperature range 473-623 K.
The data in the Table 5.12 shows that an increase of temperature
from 473 K to 523 K, increases the amount of coke deposition. Further rise
of temperature above 523 K decreases amount of coke deposition on all the
catalysts. This decrease is due to the evaporation of adsorbed volatile coke
169
Table 5.12
Coke (wt.%) formation in 1-Octene isomerisation over A and B series catalysts
WHSV=3 h"1
Catalyst
A(20)
Time on stream^ 5 h
Coke (wt.%) at different temperature
623 K
473 K
523 K
573 K
4.5
6.0
6.5
5.6
A(50)
5.4
6.3
5.5
4.6
A(100)
5.9
6.0
5.4
4.5
A(200)
5.4
6.1
5.4
Al-ZSM-5(20)
2.9
3.5
3.0
2.4
B(20)
6.4
7.7
6.8
5.9
B(50)
6.0
7.5
6.9
5.7
B(100)
5.8
7.6
6.5
5.5
B(200)
6.1
7.4
6.7
5.5
A1~MCM-41(20)
3.2
4.3
3.8
3.4
'
4.3
170
Figure 5.14
Typical IR spectrum of coke deposited A(20) catalyst
in 1-oetene isomerisation
171
molecules above 523 K The amount of coke deposit on B series catalysts is
larger than that found on A series catalysts under similar experimental
conditions. As seen in 1-hexene isomerization, the amount of coke
accumulation on boron containing catalysts in 1-octene isomerization are
more, compared with the corresponding aluminium containing catalysts in
the temperatures range studied. These observations on coke deposition over
the catalysts in 1-octene isomerization are similar to what were observed in
1-hexene isomerization. Hence explanation offered earlier holds good here
too. Thus it may be proposed that boron catalysts having only mild acidity,
are not able to crack the precoke (Cornaro et al 1987) whereas, aluminium
containing catalysts because of greater number of strong acid sites, can
crack off the precoke oligomer in the process of coke hardening. As a result,
less amount of coke deposits are observed on aluminium containing catalysts
compared to B containing catalysts.
5.5
EFFECT OF WHSV
The influence of WHSV on 1-octene conversion and product yield were
examined at various values of WHSV such as 3,6,9 and 12 h1 at 573 K. The
data obtained over the A(20), A(200) and Al-ZSM-5(20) as select cases are
presented in Table 5.13. The results obtained using B(20), B(200) and
A1-MCM-41(20) catalysts are given in Table 5.14. The data presented in the
Tables illustrate, that with an increase in WHSV, 1-octene conversion
decreases considerably over the above mentioned catalysts. This is due to
the less residence time of 1-octene molecules on the acid sites of catalysts
with increase in WHSV. Figures 5.15, 5.16, 5.17, 5.18, 5.19 and 5.20 are
plots of the selectivity of products with WHSV of A(20), A(200), Al-ZSM
5(20), B(20), B(200) and A1-MCM-41(20) catalysts respectively. The
selectivity of 2-octene and 3-oetene is found to increase and 4-octene
selectivity decreases considerably with an increase of WHSV for all the
catalysts. The decrease in the selectivity of 4-octene is attributed to the slow
rate of hydrogen shift from C4 to C3 carbon in C8 carbenium ion (II) and as
Catalyst
CN
<
13.5
79.0
73.5
SC
oo
so
04
63.0
59.0
53.5
45.6
90.0
88.2
84.6
80.2
CO
\o
OS
OO
04
CO
On
CN
to,
s
N
<
16.0
20.0
14.0
18.2
0.5
24.0
21.6
Dimethyl
hexenes
9.2
1.0
1.0
14.4
8.9
CO
oi
20.3
20.7
21.6
24.4
CN
11.5
15.9
9.6
23.7
3.0
sC
i
18.9
|
OO
19.4
16.8
13.4
h
i
9.0
17.7
18.3
17.3
16.5
17.4
13.6
9.8
16.8
22.8
20.5
heptenes
20.8
1
04
20.0
20.9
20.9
23.0
25.2
22.0
20.9
20.2
3-Octene
Product yield (wt%)
4-Octene
Methyl
Time on stream =
ro
A(200)
10.4
cO
>
ac :c
£ w
16.9
2-Octene
Octene
conversion
(wt%)
sq
co
oo
cn aT'
vC
07Z
Reaction temperature- 573 K
Effect of WHS V on the octene conversion and product yield over A (20), A(200) and
Al-ZSM-5(20) catalysts
Table 5.13
172
yr,
y-r,
04
B(20)
VO
ON
CN
VO
f
'
69.7
65.4
61.0
39.5
32.5
25.6
20.0
ON
i
i
i
i
i
1
J
12.6
11,0
9.2
<N to
OO
oo
NO
o
00
o OO UO
o
00 00 oo
On
21.0
i
i
21.2
23.2
24.4
19.9
15.7
12.7
10.3
i
'
L ...
10.2
OC
' ci m o
OO
<N
O r-
VO r<N
oo
to
2.9
-5_
CnI
18.7
i
Dimethyl
hexenes
'
19.9
.._ 4
11.6
8.0
6.0
29.0
28.2
19.2
19.0
6.0
19.0
16.5
27.0
16.4
16.7
Methyl
heptenes
3-Octene
!
Product yield (wt% )
h
2-Octene
to
75.0
..
Octene
conversion
(wt% )
1
OO
B(200)
-
1
m
<N
in
00
A1-MCM-41(20)
------
i
Catalyst
=
<> o
8'9
Time on stream
o
O
c
o
rft
YLZ
(,.M)
ASH/W
Reaction temperature=573 K
Effect of WHSV on the octene conversion and product yield over B(20), B(200) and
AI-MCM-41(20) catalysts
173
(N
174
3
6
9
12
WHSV (h1)
Selectivity (wt%
Figure 5.15 The product selectivity of 1-octene
isomerisation with WHSV over catalyst A(20)
Time on stream=1 h, T=573 K
3
6
9
12
WHSV (h~1)
Figure 5.16 The product selectivity of 1-octene
isomerisation with WHSV over catalyst A(200)
Time on stream=1 h, T=573 K
Selectivity (wt%)
175
3
6
9
12
WHSV (h1)
-p*
ru
Selectivity (wt%)
Figure 5.17 The product selectivity of 1-octene
isomerisation with WHSV over catalyst
AI-ZSM-5(20);TOS=1 h, T=573 K
3
6
9
12
WHSV (h'1)
Figure 5.18
The product selectivity of 1-octene
isomerisation with WHSV over catalyst B(20)
Time on stream=1 h, T=573 K
60
The product selectivity of 1-octene
isomerisation with WHSV over catalyst B(200)
Time on stream=1 h, T=573 K
Selectivity (wt%)
Figure 5.19
WHSV (h'1)
Figure 5.20 The product selectivity of 1-octene
isomerisation with WHSV over catalyst
AI-MCM-41(20); TOS=1 h, T=573 K
177
a result, its transformation to carbenium ion (III), an intermediate to form
4-octene decreases at a high feed flow rate. The selectivity of
methylheptenes and dimethylhexenes decreases with all the catalysts with
an increase in WHSV. This indicates that the skeletal isomerization falls
with an increase of WHSV. This is due to the slower rate of skeletal
isomerization compared with double bond isomerization and as a result the
formation of skeletal isomerization products decreases at a high feed flow
rate (less contact time). This study further indicates that lower the WHSV,
maximum is the octene conversion with more skeletal isomerization
products. At higher WHSV, octene conversion decreases and formation of
skeletal isomerization products falls to a larger extent.
5.6
EFFECT OF TIME ON STREAM
Changes in catalytic activity of A(20), A(200) and Al-ZSM-5(20)
catalysts in the isomerization of 1-octene with time on stream were
investigated at 573 K and WHSV of 3 h'1. The data on the distribution of
the products obtained are presented in Table 5.15. The results of the same
using B(20), B1200) and Al-MCM-41 are given in Table 5.16. 1-octene
conversion decreases with time on stream in all the above mentioned
catalysts and the magnitude of decrease in conversion is found to be
different for different catalysts. The trend observed in the activity of
catalysts in octene conversion during time on stream is similar to that
observed in isomerization of hexene (ie., B-MCM-41 > B-ZSM-5; Boron
catalyst > Aluminium catalyst). But the extent of decrease of conversion at
the end of 9 h time on stream is more in octene isomerization compared
with hexene isomerization in the above mentioned catalysts under identical
experimental conditions. For example at the end of 9 h time on stream,
A(20) and B(20) catalysts show 17.1% and 20.1% decrease of octene
conversion and 9.0% and 12% decrease of hexene conversion respectively.
The decrease in percentage conversion, when the feed is changed, is because
of the proximity difference in the approach of the octene and hexene
TOS (h)
-
CO
A(20)
Catalyst
Reaction temperalure= 573 K
83.6
82.0
77.5
72.5
66.5
63.5
62.0
59.9
56.9
52.5
90.0
88.5
85.6
82.0
78.0
Octene
conversion
(wt%)
t/"l
r~-
19.7
21.4
CT\
-
CO
10
03
<
o-
14.4
12.4
■
o\
-
22.1
18.0
14.8
10.9
t
24.0
0.5
2.4
3.7
r
1
CO
ON
6.2
oc
O'
r3
OC
N
1
<
2
14.1
o
SC
16.4
23.7
23.0
18.9
19.9
d
1.0
-
20.9
24.2
18.0
14.9
10.3
19.6
17.7
15.6
13.1
10.5
!
9.2
7.4
4.5
20.8
22.8
20.6
20.4
20.8
21.8
23.5
23.6
22.2
22.0
22.7
22.5
21.5
19.7
16.3
13.0
Dimethyl
hexenes
Methyl
heptenes
4-Octene
3-Octene
3h
X
16.7
10.4
13.5
16.7
19.2
22.6
16.6
18.0
19.2
19.9
20.0
9.0
2-Octene
Product yield (wt%)
WHSV =
Effect of time on stream(TOS) on octene conversion and product yield over A(20) A(200) and
Al-ZSM-5(20) catalysts
178
CO
OC
O-
CO
04
04
IT,
Os
in
r- CT\ -
m
in
<N
00
o
t-4 M3
00 oo
r- Ov - cn
OO
Tt
oc
CM
in
o
CM
r?
U
S
S
<
r-
1
1.0
18.4
14.5
m
10.5
17.5
12.4
10.4
m
24.0
r-
20.8
23.0
24.4
26.0
■
21.2
20.4
i.»
■
22.0
!
Dimethyl
hexenes
■
11.4
13.9
in
ml
0.5
-4
3.8
2.5
CT\
13.3
12.9
18.7
18.2
17.6
16.4
13.7
19.9
oc
13.1
12.6
22.2
21.3
■
i
83.5
78.6
73.0
]
16.0
16.5
19.2
2-Octene
i
1
(200)
TOS (h)
Product yield (wt%)
3-Octene
4-Octene
Methyl
heptenes
19.0
6.0
16.8
26.4
12.0
2.7
25.0
7.2
24.0
in
i
B
B (20)
Catalyst
m
i
911
Octene
conversion
(wt%)
69.7
66.0
60.0
55.5
49.6
39.0
37.5
35.2
31.8
26.0
o
r-4
CM
■
vs
WHSV = 3 IV
VC
V9Z
Rcaction tcmpcraturc= 573 K
j
1
Effect of time on stream(TOS) on octene conversion and product yield over B(20) B(200) and
A1-MCM-41(20) catalysts
179
m,
180
molecules to the active sites of the catalysts. The activity loss is further due
to coke formation that increases steadily with time on steam, thereby
poisoning the active sites. The wt % of coke formed is 7.5, 6.7, 4.2, 9.6, 8.6
and 5.6 over A(20), A(200), Al-ZSM-5(20), B(20), B(200) and A1-MCM-4K20)
catalysts respectively at the end of 9 h of time on stream at 573 K. The
wt % coke formed on all catalysts at the end of 9 h time slightly more in
oetene isomerization than found on hexene isomarisation. Figures 5.21,5.22,
5.23, 5.24, 5.25 and 5.26 are plots of the selectivity of products against time
on stream over A(20), A(200), Al-ZSM-5(20), B(20), B(200) and Al-MCM41(20) respectively. The selectivity of 2-octene and 3-octene increases and
4-octene selectivity decreases with time on stream of above mentioned
catalysts. This is attributed to the decrease of rate of hydrogen shift from
C4 carbon to C3 carbon in C8 carbenium ion (II) to form more 4-octene and
increase in the rate of deprotonation of Cg carbenium ion(II) to form 3octene as a result of increased coke formation with time on stream.
Selectivity of methylheptenes and dimethylhexenes is found to decrease with
time in both the sets of catalysts, and formation of both products are not
observed after 7 h over B(20) and B(200) catalysts. During the early stages
as the more active acid sites are free, there is high conversion of 2-octene,
3-octene and 4-octene products formed in the reaction into methylheptenes
and dimethylhexenes, but during the later stages, as the active sites are
blocked by coke deposits, linear octenes [2-octene, 3-octene and 4-octene]
formed remain without being transformed to methylheptene and
dimethyhexenes and hence the selectivity of methylheptene and
dimethylhexenes is found to decrease with an increase in time.
13
5
7
9
Time on stream (h)
The product selectivity with time on
stream in 1-octene isomerisation over A{20)
T=573K, WHSV=3 h 1
Selectivity (wt%)
Figure 5.21
13
5
7
9
Time on stream (h)
Figure 5.22
The product selectivity with time on stream
in 1-octene isomerisation over A(200)
T=573K, WHSV=3 h 1
Selectivity (wt%)
182
1
3
5
7
9
Time on stream (h)
Figure 5.23 The product selectivity with time on stream
in 1-octene isomerisation over catalyst
AI-ZSM-5 (20); T =573K, WHSV=3 h-1
1
Figure 5.24
3
5
Time on stream (h)
7
9
The product selectivity with time on stream
in 1-octene isomerisation over B(20)
T=573K, WHSV=3 h 1
183
13
5
7
9
Time on stream (h)
The product selectivity with time on stream
in 1-octene isomerisation over B(200)
T=573K, WHSV=3 h 1
Selectivity (wt.%)
Figure 5.25
1
3
5
7
9
Time on stream (h)
Figure 5.26
The product selectivity with time on stream
in 1-octene isomerisation over
AI-MCM-41 (20); T=573K, WHSV=3 h'1