Molecular sieve abstract
A molecular sieve containing composition, which can be applied
in catalytic cracking reaction for producing more ethylene and propylene,
and its preparation method. The composition contains a pentasil-type
molecular sieve having a SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio
of 15-60 prepared by activation and modification with phosphorus,
alkaline earth metal and transition metal. The composition essentially
includes 85.about.98% wt of pentasil-type molecular sieve, 1.about.10%
wt of P.sub.2 O.sub.5 0.3.about.5% wt of alkaline earth oxide,
and 0.3.about.5% wt of transition metal oxide. The molecular sieve
structure and active centers have high thermal and hydrothermal
stability. The salient feature of this composition is that when
applied as an active component of cracking catalyst for catalytic
pyrolysis process, the yield of ethylene is above 18% and the total
yield of ethylene and propylene is more than 40%.
Molecular sieve claims
Claims of the invention:
1. A pentasil-type molecular sieve containing composition which
can be applied in catalytic cracking reaction for producing more
ethylene and propylene comprising 85.about.98% wt pentasil-type
zeolite which has a SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of 15.about.60
1.about.10% wt
phosphorus (based on P.sub.2 O.sub.5), 0.3.about.5% wt alkaline
earth metal (based on its oxide), and 0.3.about.5% wt transition
metal (based on its oxide).
2. The molecular sieve composition according to claim 1 comprising
88.about.95% wt pentasil-type zeolite which has a SiO.sub.2 /Al.sub.2
O.sub.3 molar ratio of 15.about.60 2.about.8% wt phosphorus (based
on P.sub.2 O.sub.5), 0.5.about.3% wt alkaline-earth metal (based
on its oxide, and 0.5.about.3% transition metal (based on its oxide).
3. The molecular sieve composition according to claim 1 wherein
said pentasil-type zeolite is a molecular sieve having a structure
type of ZSM-5 ZSM-8 or ZSM-11.
4. The molecular sieve composition according to claim 3 wherein
said pentasil-type zeolite is a molecular sieve having a structure
type of ZSM-5.
5. The molecular sieve composition according to claim 1 3 or 4
wherein said pentasil-type zeolite has a SiO.sub.2 to Al.sub.2 O.sub.3
molar ratio of 15.about.40.
6. The molecular sieve composition according to claim 1 wherein
said alkaline earth metal is magnesium or calcium.
7. The molecular sieve composition according to claim 1 wherein
said transition metal is a metal selected from Group IB, IIB, VIB,
VIIB or VIII of the periodic table.
8. The molecular sieve composition according to claim 7 wherein
said transition metal is a metal selected from the group consisting
of Cr, Mn, Fe, Co, Ni, Cu and Zn.
9. The molecular sieve composition according to claim 8 wherein
said transition metal is a metal selected from Ni, Cu, or Zn.
10. A method for preparing said composition according to claim
1 comprising: a pentasil-type zeolite is firstly added to an aqueous
solution which contains a phosphorus-containing compound, an alkaline
earth metal compound and a transition metal compound, homogeneously
mixed and impregnated for above 0.5 hour, the obtained mixture contains(in
dry basis) 85.about.98% wt pentasil-type molecular sieves, 1.about.10%
wt phosphorus (based on P.sub.2 O.sub.5), 0.3.about.5% wt alkaline
earth metal (based on oxide), and 0.3.about.5% wt transition metal
(based on oxide); the mixture is then dried, and calcined at 450
to 650.degree. C. for 1.about.4 hours.
11. The method according to claim 10 wherein said mixture contains
(in dry basis) 88.about.95% wt pentasil-type molecular sieves, 2.about.8%
wt phosphorus (based on P.sub.2 O.sub.5), 0.5.about.3% wt alkaline
earth metal (based on oxide), and 0.5.about.3% wt transition metal
(based on oxide).
12. The method according to claim 10 wherein said pentasil-type
molecular sieve is a ZSM-5 ZSM-8 or ZSM-11 type molecular sieve.
13. The method according to claim 12 wherein said pentasil-type
molecular-sieve is ZSM-5 type molecular sieve.
14. The method according to claim 10 12 or 13 wherein said pentasil-type
molecular sieve has a silica to alumina molar ratio of 15.about.40.
15. The method according to claim 10 wherein said phosphorus-containing
compound is phosphoric acid, hydrogen phosphate or phosphate.
16. The method according to claim 10 or 15 wherein said phosphorus-containing
compound is phosphoric acid.
17. The method according to claim 10 wherein said alkaline earth
metal compound is a compound of magnesium or calcium.
18. The method according to claim 10 or 17 wherein said alkaline
earth metal compound is its nitrate or chloride.
19. The method according to claim 10 wherein said transition metal
compound is a compound containing a metal selected from the Group
IB, IIB, VIB, VIIB or VIII of the periodic table.
20. The method according to claim 19 wherein said transition metal
is a metal selected from the group consisting of Cr, Mn, Fe, Co,
Ni, Cu and Zn.
21. The method according to claim 20 wherein said transition metal
is a metal selected from Ni, Cu, or Zn.
22. The method according to claim 10 19 20 or 21 wherein said
transition metal compound is its nitrate or chloride.
23. The method according to claim 10 wherein the water to solid
weight ratio of said mixture is (1.about.3):1.
Molecular sieve description
BACKGROUND OF THE INVENTION/FIELD OF THE INVENTION
This invention relates to a pentasil-type molecular sieve containing
composition which can be applied in catalytic cracking reaction
for producing more ethylene and propylene, and its preparation method.
DESCRIPTION OF THE BACKGROUND ART
The pentasil-type molecular sieves invented by Mobil Corporation,
such as ZSM-5 (U.S. Pat. No. 3702886 1976), ZSM-8 (GB1334243A),
ZSM-11 (U.S. Pat. No. 3709979 1973) and ZSM-5/ZSM-11 (U.S. Pat.
No. 4289607 1981), after modification, are widely used in hydrocarbon
conversion reactions such as aromatic hydrocarbon alkylation, disproportionation,
isomerization, catalytic cracking, catalytic dewaxing and synthesis
of gasoline from methanol etc., in which ZSM-5 is most successfully
used.
ZSM-5 molecular sieves was synthesized formerly by using organic
amines as templates, including tetrapropylammonium, tetraethylammonium,
hexamethylenediamine, ethylenediamine, n-butylamine and ethylamine
etc. Because of the expensive cost and environmental contamination,
the method of synthesizing ZSM-5 without using organic substances
has been widely explored at the same time. For example, in EP111748A
(1984), ZSM-5 zeolite is synthesized by using water glass, aluminum
phosphate and phosphoric acid; in CN85100463A, ZSM-5 zeolite is
synthesized by using water glass, inorganic aluminum salts and mineral
acid as initial materials; in CN1058382A, rare-earth containing
ZSM-5 zeolite is synthesized by using water glass, aluminum phosphate
and mineral acids as source materials and REY or REHY as crystal
seeds. JP8571519 and JP8577123 reported a method in which ZSM-5
molecular sieve is synthesized by adding ZSM-5 as crystal seeds
under the condition that no amine is used.
In order to satisfy the different kinds of requirements in different
reactions, a lot of methods of modifying ZSM-5 molecular sieve and
their different effects are reported. For example, methods for modifying
ZSM-5 molecular sieve using a phosphorus-containing compound are
reported in U.S. Pat. No. 3972382 and U.S. Pat. No. 3965208
in which HZSM-5 with a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of 70 is
reacted with trimethyl phosphite to prepare a phosphorus containing
molecular sieve. This preparation method has several disadvantages
of complicated conditions as well as high costs, and the samples
obtained have lower reactivity than the samples not containing phosphorus,
but have an advantage of high selectivity.
U.S. Pat. No. 4365104 U.S. Pat. No. 4137195 U.S. Pat. No.
4128592
and U.S. Pat. No. 4086287 report a method of modifying ZSM-5
molecular sieve using P and Mg for the purpose of improving the
para-xylene selectivity by using the modified molecular sieve in
xylene isomerization, toluene alkylation with methanol and toluene
disproportionation reactions. The introduction of P and Mg is for
increasing the molecular sieve's shape selectivity. On the other
hand, the molecular sieve's acidity and reactivity in hydrocarbon
conversion reaction are decreased by the modification. In these
patents, P and Mg are impregnated in two steps separately. That
is the molecular sieve or molecular sieve containing catalyst is
firstly impregnated with NH.sub.4 H.sub.2 PO.sub.4 or (NH.sub.4).sub.2
HPO.sub.4 aqueous solution, and then filtered, dried, and calcined;
and secondly impregnated with Mg(NO.sub.3).sub.2 or Mg(CH.sub.3
COO).sub.2 aqueous solution, then filtered, dried and calcined,
then the P and Mg modified molecular sieve or catalyst containing
the molecular sieve is obtained. In this method, the content of
P and Mg has an uncertainty, since it depends on reaction condition
of temperature, time and calcination. At the same time, the Mg can
not be well dispersed.
In U.S. Pat. No. 4260843 a method of modifying ZSM-5 molecular
sieve with P and Be for increasing the shape selectivity is reported.
U.S. Pat. No. 4288647 has described a method of modifying ZSM-5
molecular sieves with Ca, Sr, Ba and P for improving the shape selectivity.
In these patents, the method for modifying the molecular sieve is
basically the same as the above-mentioned method with P and Mg,
but the activity of the modified molecular sieve is even further
lowered.
In above patents, the precursor of the molecular sieve is described
with a SiO.sub.2 /Al.sub.2 O.sub.3 ratio higher than 12 generally
higher than 30 (U.S. Pat. No. 3972832). The content of modification
P element is generally higher than 0.25% wt, the content of alkaline
earth element is required to be higher than 0.25% wt, and in the
range of 0.25.about.25% wt. In the examples of the embodiments,
the content of alkaline-earth element (Mg, Ca etc.) is generally
higher than the content of the P element. The object of the above
patents is mainly to increase the shape selectivity of the molecular
sieves, which are all used in isomerization and disproportionation
reactions for booming the selectivity to xylene. Generally, it is
acknowledged that after alkaline earth modification, the molecular
sieve's acidity is decreased, and its catalytic activity in hydrocarbon
conversion is decreased too.
Catalytic pyrolysis process for producing ethylene is a new way
to increase ethylene production. Conventional steam cracking method
for producing ethylene has the disadvantages of high cracking temperature
and strict requirement for the quality of the feedstock. People
hold that the production of ethylene by steam cracking method is
carried out through the mechanism of free radical reaction, so it
requires high reaction temperature. The present invention sets forth
a series of processes and catalysts in catalytic cracking for producing
lower carbon olefins in the patents such as U.S. Pat. No. 4980053
U.S. Pat. No. 5326465 U.S. Pat. No. 5358918 U.S. Pat. No.
5380690 U.S. Pat. No. 5670037 etc. In all these patents, cracking
catalysts generally contain higher silica pentasil-type zeolites
having P and rare-earth elements, and are all for increasing C.sub.3.sup.=
.about.C.sub.5.sup.= olefins production, with relatively lower ethylene
yield. Under the condition of catalytic cracking with catalyst containing
ZSM-5 molecular sieve, the C.sub.3.sup.= .about.C.sub.5.sup.= olefins
are remarkably increased because of the medium sized pore system
and higher shape selective capability, however, the reaction is
still through carbonium ion mechanism. In CN1083092A, a catalytic
pyrolysis process for producing ethylene and propylene is reported
by using cross-linked pillared clay-containing molecular sieve or
rare-earth containing pentasil-type molecular sieve as catalyst
at a temperature range of 680.about.780.degree. C. in which ethylene
can remarkably be increased. The catalyst does not contain metallic
component.
Considering that producing more ethylene needs higher reaction
and regeneration temperatures, it requires that the molecular sieve
active components must have better thermal and hydrothermal stability
with respect to both the structure and active center, that is under
the severe hydrothermal treatment condition, the molecular sieve
must maintain its high activity. In addition, it is needed to increase
the formation of carbonium ion and its direct cracking, especially
increasing the formation of primary carbonium ion and its direct
cracking, at the same time, depressing the isomerization of primary
carbonium ion to secondary or tertiary carbonium ion. Therefore,
the shape selective cracking capability of the molecular sieve is
required to be strengthened, while desirably it should have certain
dehydrogenation capability for increasing olefin yield.
SUMMARY AND OBJECTS OF THE INVENTION
The object of this invention is to provide a pentasil-type molecular
sieve containing composition which can be applied in catalytic cracking
reaction for producing more ethylene and propylene, and which has
better thermal and hydrothermal stability, and can increase the
ethylene yield further when being used in catalytic pyrolysis process,
in comparison with the catalytic materials of the prior art.
Another object of this invention is to provide a method for preparing
said pentasil-type molecular sieve containing composition.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The pentasil-type molecular sieve containing composition of this
invention comprises essentially 85.about.98% wt, preferably 88.about.95%
wt, of pentasil-type molecular sieve having a SiO.sub.2 /Al.sub.2
O.sub.3 molar ratio of 15.about.60 1.about.10% wt, preferably 2.about.8%
wt, of phosphorus (based on P.sub.2 O.sub.5), 0.3.about.5% wt, preferably
0.5.about.3% wt of an alkaline earth metal (based on oxide), and
0.3.about.5% wt, preferably 0.5.about.3% wt, of a transition metal
(based on oxide).
According this invention, the said pentasil-type molecular sieve
in said composition is ZSM-5 ZSM-8 or ZSM- 11 structure type molecular
sieve, in which the preferred molecular sieve is ZSM-5 structure
type, with a silica to alumina molar ratio of 15.about.60 preferably
15.about.40 and lower silica to Alumina ratio is beneficial for
yielding more ethylene.
The said alkaline earth metal in said composition of this invention
is preferably selected from magnesium or calcium.
The said transition metal in said composition of this invention
is a metal which has dehydrogenation function, selected from Group
IB, IIB, VIB, VIIB or VIII of the periodic table, preferably selected
from the group consisting of Cr, Mn, Fe, Co, Ni, Cu, Zn, and more
preferably selected from Ni, Cu or Zn.
In the composition of this invention, the said phosphorus, alkaline
earth metal and transition metal are introduced into said molecular
sieve by impregnating or mixing molecular sieve with their compounds.
Then the molecular sieve is dried and calcined to cause interaction.
Thereby the phosphorus, alkaline earth metal and transition metal
are solidly fixed onto the molecular sieve.
According this invention, the preparation method of said pentasil-type
zeolite containing composition of the invention comprises firstly
adding a pentasil-type molecular sieve to an aqueous solution which
contains phosphorus-containing compound, alkaline earth compound
and transition metal compound, mixing thoroughly and impregnating
for above 0.5 hour, the obtained mixture contains (in dry basis)
85.about.98% wt, preferably 88.about.95% wt of pentasil-type zeolite,
1.about.10% wt, preferably 2.about.8% wt, of phosphorus (based on
P.sub.2 O.sub.5), and 0.3.about.5% wt, preferably 0.5.about.3% wt,
of an alkaline earth metal (based on oxide), and 0.3.about.5% wt,
preferably 0.5.about.3% wt, of a transition metal (based on oxide),
the mixture is then dried, and calcined at 450 to 650.degree. C.
for 1.about.4 hours.
In the method of this invention, the said phosphorus containing
compound may be phosphoric acid, hydrogen phosphate, dihydric phosphate
or phosphate, in which phosphoric acid is preferable.
In the method of this invention, the said alkaline earth metal
compound is preferably the compound of magnesium or calcium, which
can be their nitrate, sulfate or chloride, preferably nitrate or
chloride.
In the method of this invention, the said transition metal containing
compound is the compound of a metal having dehydrogenation function
and selected from Group IB, IIB, VIB, VIIB or VIII of the periodic
table, the preferred metal compound is a compound selected from
the compound of Cr, Mn, Fe, Co, Ni, Cu or Zn, the more preferred
compound is selected from the compound of Ni, Cu or Zn, which can
be their nitrate, sulfate or chloride, preferably their nitrate
or chloride, most preferably their chloride.
In the method of this invention, the said mixture has a water to
solid weight ratio of (1.about.3):1.
In the method of this invention, the said calcination can be proceeded
in air or in steam.
In comparison with the method of prior art (stepwise impregnation
and calcination in U.S. Pat. No. 4137195), the method of this
invention only needs one-step-through impregnation and calcination,
it does not need series of steps for impregnation and calcination.
So this method has not only simplified the preparation process,
but also reduced the energy consumption and preparation cost; moreover,
the product obtained has adequate hydrothermal stability, and when
used in catalytic pyrolysis cracking, its ethylene yield is higher
than that of the method of prior art.
In the reports of earlier patents, the introduction of activation
element such as alkaline earth metal or transition metal can enhance
the shape selective property, but the reaction activity is decreased.
This is because the amount of activation element introduced is restrained
by the zeolite. In the course of research for this invention, it
is found that the amount of activation element introduced is correlated
with the silica to alumina ratio of the zeolite. That is, the lower
is the silica to alumina ratio, the higher is the capacity of activation
elements to be introduced. For molecular sieve having a lower silica
to alumina ratio, when a larger amount of activation elements is
introduced, it can still maintain high reaction activity. In view
of the fact that a higher the amount of activation element is introduced,
the stronger its shape selective property will be, hence, selecting
a zeolite with lower silica to alumina ratio is beneficial for producing
ethylene in catalytic pyrolysis cracking reaction. However, in the
prior art, it is generally emphasized that higher silica to alumina
ratio is of benefit to the reaction. This is a special feature of
this invention.
In this invention, it is found that, after the activation element,
such as alkaline earth metal, is introduced, the number of Bronsted
acid sites of the zeolite can be reduced, while the Lewis acid sites
is relatively increased. The increase of Lewis acid sites is beneficial
for ethylene production.
The transition metals such as Ni, Co, Zn, Cu, Cr, Mn introduced
into pentasil-type molecular sieve are generally used as an active
component for hydrogenation or dehydrogenation or aromatization
reactions. This is because these elements have stronger hydrogen
transfer property. In the catalytic cracking reaction for producing
ethylene, the introduction of these elements can enhance the hydrogen
transfer ability of the molecular sieve but is not of benefit to
increasing the selectivity to olefin products. In our research work,
it is found that the hydrogen transfer capability of transition
metal is apparently suppressed in the presence of phosphorus element,
and the selectivity to olefin, especially to ethylene and propylene,
are increased, because of its dehydrogenation activity. In this
invention, therefore, on the basis of the introduction of phosphorus
and alkaline earth elements into the pentasil-type zeolite, the
transition metals such as Ni, Zn, Cu are further introduced for
improving the ethylene yield. At the same time, the presence of
phosphorus can enhance the hydrothermal stability of the molecular
sieve. This is another distinct feature of this invention.
In conclusion, this invention has provided a molecular sieve containing
composition, by which the lower carbon olefins especially ethylene
and propylene produced by catalytic pyrolysis cracking reaction
can be remarkably increased in comparison with the prior art. In
addition, the said composition has good hydrothermal stability.
The ZSM-5 molecular sieve obtained by synthesis belongs to orthorhombic
crystal system. By inorganic ammonium exchange, NH.sub.4 ZSM-5 can
be prepared therefrom, and the HZSM-5 is obtained after calcination
at 500.about.600.degree. C. In these preparation processes, the
structural symmetry of the molecular sieve is found fundamentally
to be unchanged. Under severe high temperature hydrothermal treatment,
change in the structure symmetry of the ZAM-5 molecular sieve will
occur, in which, its critical feature is that the peak at 2.theta.=24.4.degree.
in x-ray diffraction (XRD) pattern becomes widened or even split.
This change in structural symmetry is coincidental with the decrease
of reaction activity of the molecular sieve in cracking reaction.
So, in the examples of this invention and the comparative examples,
the hydrothermal stability of active centers can be judged by x-ray
diffraction (XRD) pattern. At the same time, the activity is evaluated
with pulse-micro-reaction using tetradecane (nC.sub.14 )as reactant.
In this invention, the molecular sieve materials used in the examples
and comparative examples and their properties are listed as follows:
1. ZSM-5A, produced by the Zhoucun Catalyst Plant of the Qilu Petrochemical
Co., synthesized with ethylamine as organic template, detemplated
by calcination. Its silica to alumina molar ratio is 52.0 Na.sub.2
O content is 0.10% wt after NH.sub.4.sup.+ exchange.
2. ZSM-5B, produced by the catalyst plant of Changling Refinery,
with a silica to alumina molar ratio of 25.0 and a Na.sub.2 O content
of 0.10% wt after NH.sub.4.sup.+ exchange.
3. ZSM-5C, synthesized in our laboratory (example 1), with a silica
to alumina ratio of 19.0 and a Na.sub.2 O content of 0.05% wt after
NH.sub.4.sup.+ exchange.
In this invention, the chemical compositions of molecular sieves
used in the examples and comparative examples are determined by
x-ray fluorescence (XRF) spectrometry.
Further explanation is made by the examples as follows in this
invention.
EXAMPLE 1
This example illustrates the synthesis of the ZSM-5 zeolite having
a lower silica to alumina ratio used in this invention.
24.6 g of sodium meta-aluminate was dissolved in 667 g of decationized
water, then 71.7 g of H.sub.3 PO.sub.4 (85 wt %) was added with
stirring. After homogeneously stirring, the solution was added into
643 g of water glass (SiO.sub.2 28 wt %, Na.sub.2 O 9.0 wt %) and
stirred for 4 hours, then 19.5 g of ZSM-5 molecular sieve (produced
by Zhoucun catalyst plant, SiO.sub.2 /Al.sub.2 O.sub.3 =52.0) was
added as crystal seeds. After continuously stirring for 2 hours,
the mixture was transferred to a stainless steel autoclave, crystallized
under stirring at 175.degree. C. for 15 hours and then cooled to
room temperature. The crystallized product was then filtered out,
washed and dried at 120.degree. C., and the ZSM-5C sample was obtained.
The relative crystallinity of the sample (compared to ZSM-5A) was
92% given by XRD analysis.
This sample was exchanged under a 1:1:20 weight ratio of molecular
sieve to ammonium nitrate to decationized water at 90.degree. C.
for 2 hours, then filtered and water washed, the filter cake was
exchanged again under the same conditions, and dried at 120.degree.
C. The ammonium-type ZSM-5C sample was obtained with a Na.sub.2
O content of 0.05% wt. |