Note: Descriptions are shown in the official language in which they were submitted.
LAB 104
, .
DEHYDROGENATION PROCESS
p..
Background of the Invention
The present invention relates to a process for
producing vinyl aromatic hydrocarbons by dehydrogenation of
alkylaromatic hydrocarbons. More specifically, the -
invention is directed to the dehydrogenation of
5 ethylbenzene and ethyltoluene to vinylbenzene and r-
vinyltoluene respectively.
Catalytic dehydrogenation of alkyd or
dialkylaromatic hydrocarbons is one of the commercial
processes presently employed for the manufacture of the
10 vinyl aromatic hydrocarbons, products which are extensively
used for the further production or homopolymers and
copolymers. In these dehydrogenation processes, the feed
mixture contains alkyd or dialkyl aromatic hydrocarbons and
an inert delineate in the vapor phase. The dehydrogenation
15 reaction may be executed by passing the feed mixture either L
through a single reactor or through two successive
reactors. In this latter case, the reaction mixture is
reheated before entering the second reactor. The inert
diluentj preferably steam, supplies heat to the endothermic
20 reaction and favors the production of the vinyl aromatic
hydrocarbons. For example, ethylbenzene is vaporized,
heated and then passed together with steam through the
dehydrogenation reactor(s) containing a suitable catalyst.
The feed mixture is generally introduced at a temperature
i08~ -
higher than 600~C resulting in some thermocracking of
ethylbenzene. The dehydrogenation reaction produces
styrenes and additional by-products including Bunsen,
Tulane, tar products and coke. The final product yield is
accordingly affected by the production of these unwanted
by-products.
The catalysts used in these dehydrogenation
processes generally contain one or more oxides of iron,
chromium or zinc compound, and a smaller amount of an
alkali metal oxide, particularly potassium oxide; it has
been found that potassium oxide promotes the removal of
coke and tars by reaction with steam through a water-gas
reaction, and mitigates therefore a carbon build-up on the
catalyst surface. In recent years, improved catalysts of
this general type consisting of mixtures of metal oxides
have been described. They provide a selectivity with
respect to the vinyl aromatic hydrocarbon in the range from
about 85 mole percent to about 95 mole percent, depending
on the feed, for a conversion of the ethyl aromatic
hydrocarbon in the I to 45 mole percent range after the
first step, and about 60 to 70 mole percent after the
second step.
Many attempts to improve the overall yield of
vinyl aromatic hydrocarbons by catalytic dehydrogenation of
hydrocarbon feed stocks have been reported. These recent
works are more particularly directed to the use of new
catalysts in order to improve this selectivity; for
example, it was expected that aluminosilicate zealots
would be more selective than the previously used catalysts.
30 Previous experiments have been conducted utilizing these i
I.
r
I
catalysts, typically Nix and Nay zealots impregnated with
chromium compounds, for the dehydrogenation of
ethylbenzene; however, their selectivity with respect to
vinylbenzene did not exceed 60%.
A crystalline silica composition having a uniform
pore structure but not exhibiting ion exchange properties
was disclosed in US. Patent 4,061,724 by Grove, wherein
the crystalline silica was useful for the separation of
p-xylene from o-xylene, m-xylene and ethylbenzene, and for
10 selectivity absorbing organic materials from water. L_
Summary of the Invention
. . .
In order to overcome the problems, including lack
of selectivity, of the previous dehydrogenation processes,
15 there is provided in accordance with the present invention,
a process for producing vinyl aromatic hydrocarbons by r
dehydrogenation of the corresponding alkyd aromatic
hydrocarbons. The process comprises passing the alkyd
aromatic hydrocarbons on a crystalline silica which has
20 been calcined under inert atmosphere and which contains
from about 0.05 to about 1% yo-yo metal oxides, at a or
temperature between about 500C and about 650C.
The crystalline silica is obtained by
hydrothermal crystallization of a reaction mixture
25 containing water, a source of silica, and a qua ternary
R4 I.
ammonium salt having the formula: R1 - N - R3 I I
R2 I.
or a qua ternary phosphonium salt having the formula:
PHI+ X , wherein R1, R2, R3 and R4 are the
--4--
same or different alkyd radicals, and X is the radical of a
monovalent acid, at a pi between about 7 and 14, to form a
hydrous crystalline precursor. The hydrous crystalline
precursor is washed with water and then washed with a __
5 strong acid, then dried and calcined in an inert
atmosphere. The crystalline silica is not submitted to any
further washing in order to obtain an amount of alkali
metal oxides remaining therein between 0.5% and 1 wit %. -
Those familiar with the art of catalysis know
10 that even minor variations in compositions and/or in method
of their manufacture may result in significant and
unexpected variations in the behavior of the catalyst for
a given reaction. The main factors to be considered,
however, are the activity, the selectivity and the
15 stability of the catalyst. An object of the present
invention, therefore, is to provide a novel process for r
producing vinyl aromatic hydrocarbons by dehydrogenation of
the corresponding alkyd aromatic hydrocarbons to obtain
superior overall yields of vinyl aromatic hydrocarbons.
20 One of the more specific objects is to provide a catalytic
process of improved activity and selectivity with respect
to the dehydrogenation of alkyd or dialkyl aromatic
hydrocarbons to the corresponding vinyl hydrocarbons. A
further object of the invention is to increase the
25 production capacity of a given dehydrogenation unit. The jar.
particular object of the invention is to provide an process
for dehydrogenating ethylbenzene and ethyltoluene to obtain
higher overall yields in vinylbenzene and vinyltoluene.
- I
--5--
Detailed Description of the Preferred Embodiments
In accordance with this invention, it has been
found that improved overall yields of vinyl aromatic
hydrocarbons are achieved when the corresponding alkyd
aromatic hydrocarbons are contacted under dehydrogenation
reaction conditions with a crystalline silica which
contains a residual amount of alkali metal oxides and which
has been calcined under an inert atmosphere. Particularly, --
high overall yields are obtained in accordance with the
process of the present invention when ethylbenzene or
ethyltoluene are dehydrogenated in the presence of the
crystalline silica catalyst. The process of the present
invention may also be applied to the dehydrogenation of
other alkyd or dialkyl aromatic hydrocarbons, such as
15 isopropylbenzene or diethylbenzene. _
The crystalline silica is prepared by I-
hydrothermal crystallization of a reaction mixture
containing water, a source of silica, and a qua ternary
ammonium salt having the formula:
R4
R1 - N - R3 X
R2 I_
or a qua ternary phosphonium compound having the formula:
PHI X , wherein R1, R2, X3 and I are the
same or different alkyd radicals, and X is the radical of a
monovalent acid, at a pew between about 7 to about 14, to
form a hydrous crystalline precursor. The hydrous
crystalline precursor is washed first with water and then
with a strong acid such as hydrochloric acid, then dried,
and finally calcined in an inert atmosphere at a
temperature between about 450C and about 900C. After
so
--6--
calcination, the attained crystalline silica is not
submitted to any further washing to avoid loss of the
residual alkali metal oxide located therein.
The alkali metal oxide may result from the silica
itself. In other words, the alkali metal oxide may exist
in sufficient quantities as an impurity in the source of
silica itself without further treatment. On the other
hand, the source of silica may be an alkali metal silicate
formed by reacting an alkali metal hydroxide with colloidal
10 silica. In most cases, the residual alkali metal appears r-
as impurities in the crystalline product. Its removal can
be carried out if desired by washing with a strong acid
after the calcination step or by the usual ion exchange
technique using an NH4~ salt. In the process of the
15 present invention, however, it has been found that a
limited amount of alkali metal should remain in the
crystalline structure. Generally, the residual amount of
alkali metal oxide does not exceed about I based on the
weight of the crystalline silica, and is preferably between
about 0.05 and about 0.5%. It has also been observed that
a crystalline silica having too high a residual amount of
alkali metal oxide therein leads to catalysts of less r-
thermal stability and therefore, lower dehydrogenation I;
activity. Surprisingly, it has been found that calcination t
25 under an inert atmosphere has a stabilizing effect on the
crystalline structure while such an effect does not appear
with calcination under an oxidizing atmosphere.
In US. Patent No. 4,061,724 by Grove,
an alkylonium salt was
30 utilized in the preparation of a silicalite catalyst having
i
--7--
no ion exchange characteristics. The silicalite of the
Grove patent exhibited hydrophobic tendencies making it a
possible candidate for selective removal of organic from I-
waste water. __
In order to prepare the crystalline silica of the
present invention, the reaction mixture will contain from
150 to 700 moles water, from 13 to 50 moles of
non-crystalline Sue, and from 0.3 to 6.5 moles of MOO
wherein M is an alkali metal, per mole of qua ternary
10 ammonium salt or qua ternary phosphonium salt The reaction r-
mixture is maintained at a temperature from about 100 to
250C under autogeneous pressure until crystals of the
silica are formed, ordinarily in about 50 to about 150
hours. The qua ternary salt of the present invention may be
15 produced in situ; for example, a qua ternary ammonium
chloride may be generated from a alkyd chloride and a r
tertiary amine.
The crystalline silica of the present invention,
prepared in accordance with the particular conditions
20 described above, has an X-ray powder diffraction pattern
which is similar to the crystalline silica described in
US. Patent No. 4,061,724. The following table lists the r
data representing the X ray powder diffraction pattern of a I-
typical crystalline silica compound of the present
invention containing from 20 to 40 moles of Sue and from
about 0.05 to about 1 moles of NATO per mole of
qua ternary ammonium salt.
so
Table I
Distance Relative distance Relative
Angstroms Intensity (Angstroms) Intensity
11.16 47 3.44 13
510.02 29 3.35 8
9.78 14 3.33 10
9.05 3 3.26 4
7.48 11 3.19
7.09 4 3.16 2
106.72 7 3.06 12
6.35 13 2.99 9
6.06 2.96 4
5.99 9 2.88
5.72 7 2.80
155.58 8 2.74 4
5.38 2 2.61 5
5.13 4 2.58 2
4.99 6 2.52 2
4.62 7 2.50 5
204.45 8 2.42 2
4.38 11 2.41 3
4.26 12 2.01 7 r
4.10 3 2.00 9 I
4.01 7 1.97
253.86 100 1.96 2
3.76 27 1~92 2
3-73 47 1.88 3
3~66 35
3.60 2
303.49 3
SLY
r
I
It has been found that such a crystalline silica
has unexpected catalytic properties in the dehydrogenation --
of alkyd or dialkyl aromatic compounds. The thermal
stability of the crystalline silica of the present
invention is an important factor in the product yield from
dehydrogenation of alkyd aromatics because it allows the
catalyst to be used for longer periods of time, for example
more than 500 hours. The further advantage of such a --
crystalline silica resides in the fact that it is thermally
10 stable up to temperatures as high as 900C. I-
Moreover, the yields obtained with the catalyst
of the present invention are really quite surprising since
the literature teaches that catalytic activity of zeolitic
type catalysts is due to the presence of aluminum atoms in
the structure of the zealot and specifically to the number
of aluminum atoms present. It is known Pi
that a
crystalline silica or "silicalite" similar to the one
described in US. Patent 4,061,724, but which has been- -
calcined in an oxidizing atmosphere and washed with
hydrochloric acid to remove the alkali metal oxide
impurities, has catalytic activity in aromatization
reactions, but that overall yields are improved when the
25 crystalline silica/"silicalite" is used in conjunction with I-
Allah.
The conditions under which dehydrogenation takes
place in the present invention can be the conditions under
Lo
--10--
which normal vapor phase catalytic dehydrogenation
reactions are performed. Thus, reaction temperatures
should be between about 550C and 650C, and preferably
between about 580C and 630C. Similarly, pressures may
vary widely and can range from sub-atmospheric, for example
0.1 atmosphere, to super-atmospheric, for example 50
atmospheres. Preferably, however, pressures may range from
about 0.3 to 3 atmospheres, and more preferably from about
0.4 to 1 atmosphere. The dehydrogenation reaction is
generally carried out in the presence of a gaseous delineate
which is employed to reduce the partial pressure of the
reactants and to control their residence time in the
reactor(s). Illustrative of the delineate gases that may be
used are helium, nitrogen, carbon dioxide, steam, or
mixtures thereof; preferably, however, the delineate is steam
or carbon dioxide or mixtures of steam and carbon dioxide. I_
In a preferred embodiment of the process of the
present invention, carbon dioxide is used as a delineate. It
has been found that use of carbon dioxide results in
improved conversion of the alkyd aromatic hydrocarbons.
While not wishing to be bound by the theory of operation,
it is believed that the use of carbon dioxide increases
yields by an inverse water gas shift reaction.
The molar ratio of gases delineate to alkyd or
25 dialkyl aromatic compounds may vary over a wide range from k`
at least about 1 mole to about 25 moles of delineate per mole
of alkyd or dialkyl aromatic compounds; however, a molar
ratio of from about 5 moles to about 16 moles of delineate to
alkyd or dialkyl aromatic compound is more generally used.
30 When operating under low pressures, the molar ratio is I?
L
~L5~8~
-11
preferably between about 5 and about 10.
The rate of feeding the alkyd or dialkyl aromatic
hydrocarbon and delineate over the catalyst bed, or in other
words, the liquid hourly space velocity (LHSV) (volume of
5 feed per volume of catalyst per hour) may vary widely from
about 0.01 Jo 1Ø
An advantage of the catalyst of the present
invention resides in the fact that it is easily regenerated
according to known methods in the art; particularly the
10 regeneration includes heating in a nitrogen diluted air
stream at 600C until exothermicity has ceased.
The following examples are presented in order to
be illustrative of the present invention without limiting
its scope.
Example 1 pi
A catalyst was prepared by mixing 79.2 grams of
colloidal silica containing 0.8 wit% NATO in 250 grams
water with 18 grams of (C3H4)4N+Br in 54 grams
HO; 8.4 grams Noah in 30 grams HO was added to the
mixture. During the stirring, the pi of the mixture has
varied from 11 to 9. After stirring, the mixture was r
heated at 150C in an autoclave for 3 days. The resulting
crystalline silica was calcined at 600C under an inert
25 atmosphere of No. The crystalline silica had a residual
amount of NATO or 0.5 wit%.
This catalyst was charged in a reactor wherein a
mixture of ethylbenzene and carbon dioxide delineate was fed
in a molar ratio of ethylbenzene to carbon dioxide of 1:16.
30 The dehydrogena~ion reaction was effected under atmospheric
! '.
ooze
-12-
pressure, at a temperature of 600C with a LHSV of 0.1.
The following results were obtained 50 hours after
startup:
conversion of ethylbenzene (wit %): 85.18
selectivity in styrenes (wit %): 98.0
Example 2
The catalyst prepared in Example 1 was used for
dehydrogenation of ethylbenzene under the following
conditions:
gaseous delineate: COY
molar ratio of diluent/ethylbenzene: 13
temperature: 600DC
pressure: atmospheric
LHSV: 0.1
The following results were obtained:
Time after start-up (his): 170 250
Conversion of ethylbenzene (%) 62.2 61.2
Selectivity in styrenes (~) 97.2 97.3
Comparison Example PA
By way of comparison, a catalyst such as that I_
prepared in Example 1 was used, except that the catalyst I.
was calcined for 1 hour at 600C in an oxidizing atmosphere I-
25 and thereafter washed with hydrochloric acid. The residual
amount of NATO was less than 160 Pam. This catalyst was
tested for dehydrogenation of ethylbenzene under the
following conditions:
Gaseous delineate: COY .
molar ratio of diluent/ethylbenzene: AYE 19(B)
i08~
-13-
temperature: 600C
pressure: atmospheric
LHSV: 0.1
The following results were obtained:
(A) (B)
Time after start-up (his) 22 47
conversion of ethylbenzene (%) 94.6 93.3
selectivity to styrenes (%) 11.0 12.7
selectivity to Bunsen (%) 85.8 82~9
The above example indicates that catalysts
prepared in the above manner show excellent dealkylating
properties but are not effective for dehydrogenation
purposes.
15 Comparison Example 2B I:
By way of comparison, a catalyst such as the one
prepared in Example 1 was used, except that it was not
washed with hydrochloric acid, and it was calcined for 1
hour at 600C in an oxidizing atmosphere. The residual
20 amount of NATO was 0.5%.
The catalyst was tested for the dehydrogenation
of ethylbenzene under the following conditions:
Gaseous delineate: COY I--
molar ratio of diluent/ethylbenzene: 16 r
temperature: 600C k
pressure: atmospheric
LHSV: 0.13
The following results were obtained:
Time after start-up (his): 116
Conversion of ethylbenzene(%): 9.21
Selectivity to styrenes (%): 86.3
,.
~5~8~
-14-
Although the catalyst showed some dehydrogenation
properties, the results showed significantly less
conversion than in Examples 1 and 2.
Comparison Example 2C
By way of comparison, the procedure described in
Example 1 was used for preparing a catalyst, except that
the starting colloidal silica contained 2 wit % NATO. As
a result, the obtained catalyst had a content in residual
NATO (1.38 wit %) which was too high. It was tested or
the dehydrogenation of ethylbenzene; it was observed that
the catalyst had no more activity after 102 his.
Comparison Example ED
By way of further comparison, a catalyst prepared
by the method described in Example 1 was tested. After r
calcination under inert atmosphere, this catalyst was
washed with hydrochloric acid in order to remove the alkali
metal oxide impurities. The resulting catalyst had no
significant dehydrogenation activity.
I"
Example 3 r
the catalyst prepared in Example 1 was used for t
dehydrogenating ethylbenzene under the following
conditions:
gaseous delineate: No
molar ratio of diluent/ethylbenzene: 15
temperature: 600C t
pressure: atmospheric
LHSV: 0.1
After 101 hours run, the conversion of L
Jo 5
ethylbenzene was only of 23.28%. At this time the No
delineate was replaced by COY; the molar ratio of
CO2/ethylbenzene was lo. Seventeen hours after the I'.
substitution of delineate, the conversion was about 32%, with
a selectivity to styrenes of 97.3%; 78 hours after the
substitution of delineate, the conversion was about 62.21%
while the selectivity to styrenes was 97.2%, and 460 hours
after this substitution the selectivity to styrenes was
still higher than 90%.
This example shows the advantage of using COY _-
as delineate in the dehydrogenation reaction of ethylbenzene
to styrenes
Example 4
A catalyst was prepared in accordance with the
method described in Example 1, except that KOCH was added
instead of Noah.
The resulting crystalline silica was calcined at
600C under an inert atmosphere of COY. The crystalline
silica had a residual amount of KIWI of 0.66 wit %.
The catalyst was used for dehydrogenating
ethylbenzene under the following conditions: _
Gaseous delineate: COY
Molar ratio diluent/ethylbenzene: 7 r
Temperature: 600C 'I.
L~SV: 0.2 ,.
Pressure: atmospheric
After 27 hours run, the conversion of
ethylbenzene was 20.5%. The selectivity to styrenes was
LO
.
-16-
91.3%. After regeneration in a No stream containing 2%
oxygen, selectivity increased to 94.4~ after 64 hours run.
I'
Example 5 ,_
A catalyst was prepared in accordance with the
method described in Example 1, except that KOCH was added
instead of Noah. The resulting crystalline silica was
washed with Hal and then calcined at 600C under an inert
atmosphere of COY. The crystalline silica had a residual
amount of KIWI of 0.66 wit %.
The catalyst was used for dehydrogenating
ethylbenzene under the following conditions:
Gaseous delineate: COY
Molar ratio diluent/ethylbenzene: 9
LHSV: 0.2
Temperature: 600C
Pressure: atmospheric
After 74 hours run, the conversion of
ethylbenzene was 30.4~ with a selectivity to styrenes of
95.3%-
While the present invention has been described in
various embodiments and illustrated by numerous examples,
the person of ordinary skill in the art will appreciate I:
that various modifications, substitutions, omissions, and
changes may be made without departing from the spirit
thereon.
