Molecular sieve abstract
A method is provided for converting oxygenates, e.g., methanol,
to olefins, e.g., ethylene and propylene, comprising contacting
said oxygenates and an aromatics co-feed, e.g., xylenes, with a
framework gallium-containing molecular sieve catalyst comprising
pores having a size ranging from about 5.0 Angstroms to about 7.0
Angstroms, e.g., ZSM-5 under production conditions effective to
produce olefins. A catalyst composition is also provided, comprising
a ZSM-5 zeolite-bound ZSM-5 zeolite having a bound zeolite of framework
Ga-containing zeolite having a Si/Ga molar ratio ranging from 5
to 500 and a binder of Ga-modified, e.g., Ga-exchanged and/or Ga-impregnated,
zeolite having a Si/Ga molar ratio ranging from 5 to .infin..
Molecular sieve claims
What is claimed is:
1. A method for converting oxygenates to olefins comprising contacting
said oxygenates and an aromatics co-feed with a framework gallium-containing
molecular sieve catalyst comprising pores having a size ranging
from about 5.0 Angstroms to 7.0 Angstroms, under conversion conditions
effective to produce olefins.
2. The method of claim 1 wherein said molecular sieve catalyst
is selected from the group consisting of ZSM-5 ZSM-11 ZSM-12
ZSM-23 ZSM-35 ZSM-48 and MCM-22.
3. The method of claim 1 wherein said molecular sieve catalyst
is selected from the group consisting of ZSM-5 and ZSM-11.
4. The method of claim 1 wherein said molecular sieve catalyst
comprises ZSM-5.
5. The method of claim 1 wherein said oxygenates are selected from
the group consisting of methanol, ethanol, n-propanol, isopropanol,
C.sub.4-C.sub.20 alcohols, methyl ethyl ether, di-methyl ether,
di-ethyl ether, di-isopropyl ether, methyl isopropyl ether, ethyl
isopropyl ether, di-methyl carbonate, carbonyl compounds, and mixtures
thereof, and said aromatics co-feed comprises aromatic compound
which can diffuse into channels or cages of said catalyst together
with oxygenate and are selected from the group consisting of benzene,
toluene, xylenes, light reformates, full-range reformates or any
distilled fraction thereof, coker naphtha or any distilled fraction
thereof, FCC naphtha or any distilled fraction thereof, steam crack
naphtha or any distilled fraction thereof and coal derived aromatics.
6. The method of claim 1 wherein said oxygenates are selected from
the group consisting of methanol and dimethyl ether and said aromatics
co-feed is selected from the group of aromatic compounds consisting
of toluene and xylenes.
7. The method of claim 2 wherein said oxygenates comprise methanol
and said aromatics co-feed comprises xylenes.
8. The method of claim 5 wherein the molar ratio of oxygenate to
aromatic compound is greater than 0.1:1 and less than 300:1.
9. The method of claim 1 wherein said conversion conditions comprise
a temperature of from about 100.degree. C. to about 600.degree.
C., a pressure of from 1 psia to 200 psia (6.9 to 1380 kPa), and
a weight hourly space velocity in the range of from about 0.01 to
about 500 hr.sup.-1.
10. The method of claim 2 wherein said conversion conditions include
a temperature of 350.degree. C. to 480.degree. C., a pressure of
from about 5 psia to 100 psia (34 kPa to 680 kPa), and a weight
hourly space velocity in the range of from about 2 to about 100
hr.sup.-1.
11. The method of claim 1 wherein said conversion conditions are
effective to provide an ethylene/propylene molar product ratio ranging
from 0.1 to 7.
12. The method of claim 2 wherein said conversion conditions are
effective to provide an ethylene/propylene product ratio of at least
1.
13. The method of claim 1 wherein said catalyst is a zeolite bound
zeolite.
14. The method of claim 1 wherein said catalyst is a zeolite bound
zeolite having a bound framework Ga-containing zeolite having a
Si/Ga molar ratio ranging from 5 to 500 and a binder of framework
Ga-containing zeolite having a Si/Ga molar ratio ranging from 5
to .infin..
15. The method of claim 1 wherein said catalyst is a zeolite bound
zeolite having a bound Ga-modified zeolite having a Si/Ga molar
ratio ranging from 5 to 500 and a binder of Ga-modified zeolite
having a Si/Ga molar ratio ranging from 5 to .infin..
16. The method of claim 1 wherein said catalyst comprises silicoaluminophosphate.
17. A method for converting methanol and/or dimethyl ether to a
product containing C.sub.2 and C.sub.3 olefins which comprises the
step of contacting a feed which contains methanol and/or dimethyl
ether with a catalyst comprising a gallium-modified ZSM-5 porous
crystalline material, said contacting step being conducted in the
presence of an aromatic compound under conversion conditions including
a temperature of 350.degree. C. to 480.degree. C. and a methanol
and/or dimethyl ether partial pressure in excess of 6.9 kPa, and
the aromatic compound being capable of alkylation by the methanol
and/or dimethyl ether under said conversion conditions.
18. The method of claim 17 wherein said catalyst comprises zeolite-bound
zeolite having a bound framework Ga-containing zeolite having a
Si/Ga molar ratio ranging from 5 to 500 and a binder of framework
Ga-containing zeolite having a Si/Ga molar ratio ranging from 5
to .infin..
19. The method of claim 18 wherein said catalyst comprises zeolite-bound
zeolite having at least one component selected from the group consisting
of bound Ga-modified zeolite having a Si/Ga molar ratio ranging
from 5 to 500 and a binder of Ga-modified zeolite having a Si/Ga
molar ratio ranging from 5 to .infin..
20. A catalyst composition comprising a ZSM-5 zeolite-bound ZSM-5
zeolite having a bound framework Ga-containing zeolite having a
Si/Ga molar ratio ranging from 5 to 500 and a binder of framework
Ga-containing zeolite having a Si/Ga molar ratio ranging from 5
to .infin..
21. The catalyst composition of claim 20 wherein said catalyst
comprises at least one component selected from the group consisting
of bound Ga-modified ZSM-5 zeolite having a Si/Ga molar ratio ranging
from 5 to 500 and a binder of Ga-modified ZSM-5 zeolite having a
Si/Ga molar ratio ranging from 5 to .infin..
Molecular sieve description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method for converting
oxygenate, such as methanol and/or dimethyl ether, to olefins in
a reactor over a framework gallium-containing medium pore molecular
sieve catalyst such as ZSM-5 or ZSM-11 wherein oxygenate can be
co-fed with aromatics. The method is especially useful for increasing
ethylene/propylene product ratio.
[0003] 2. Description of the Background Art
[0004] A remarkable growth in the production of synthetic fibers,
plastics and rubber has taken place in recent decades. This growth,
to a very large extent, has been supported and encouraged by an
expanding supply of inexpensive petrochemical raw materials such
as ethylene, propylene, as well as four and five carbon olefins.
Along with this growth has been an increasing demand for alkylate,
made by reacting olefins with isobutane, for use as a high octane
gasoline component.
[0005] Burgeoning demand for olefins, particularly ethylene, propylene
and butenes, has of course led to periods of shortage, causing substantial
price increases in the feedstocks to various commercialized technologies.
These feedstocks are largely C.sub.2 to C.sub.4 paraffins co-produced
with natural gas and/or paraffinic straight run naphtha. Such feedstocks
can be substantially more expensive than methane, making it desirable
to provide efficient means for converting methane to olefins.
[0006] Conversion of methane to methanol followed by conversion
of methanol to light olefins is among the most economic routes to
make light olefins from methane. In this respect, it is known that
methanol or dimethyl ether can be catalytically converted to olefin-containing
hydrocarbon mixtures by contacting under certain conditions with
particular types of crystalline zeolite materials. U.S. Pat. Nos.
4025575 and 4038889 for example, both disclose processes whereby
methanol and/or dimethyl ether can be converted to an olefin-containing
product over a Constraint Index 1-12 zeolite catalyst, particularly
ZSM-5. In fact, ZSM-5 converts methanol and/or dimethyl ether to
hydrocarbons containing a relatively high concentration of light
olefins with prolonged catalyst lifetime before catalyst regeneration
becomes necessary. U.S. Pat. No. 4311865 teaches the use of the
medium pore zeolite, ZSM-5 (approximately 5.5 Angstroms) pore size,
which is ion-exchanged with cobalt, and then calcined to produce
a catalyst, and has been used to convert methanol to hydrocarbons
(including olefins). This process uses ion-exchange to add the metal
to the medium pore molecular sieve. Despite the durability of these
medium pore size catalysts, they exhibit a low selectivity for ethylene
when converting oxygenates. For example, HZSM-5 can exhibit ethylene
selectivity of less than 5%.
[0007] It has also been reported that other types of zeolite catalysts
can be used to convert methanol and/or dimethyl ether to olefin-containing
hydrocarbons products containing higher proportions of light olefins
than previously obtained with ZSM-5. For example, U.S. Pat. No.
4079095 discloses that zeolites of the erionite-offretite-chabazite
type, and especially ZSM-34 can promote conversion of methanol
and/or methyl ether to products comprising a major amount of ethylene
and propylene. However, while erionite-offretite-chabazite type
catalysts are highly selective to light olefins production, such
smaller pore zeolites tend to age rapidly in comparison to ZSM-5
when used for methanol/dimethyl ether conversion, perhaps owing
to the rapid buildup of coke inside the zeolite cages, which blocks
the accessibility of methanol feed to the acid sites contained therein.
Small pore catalysts such as SAPO-34 have been used to convert methanol
to olefins, as described in an article by T. Inui, "Structure-Reactivity
Relationships in Methanol to Olefins Conversion in Various Microporous
Crystalline Catalysts, Structure-Activity and Selectivity Relationships
in Heterogeneous Catalysts", pages 233-42 Elsevier Science
Publishers, B. V., Amsterdam (1991). U.S. Pat. No. 5962762 to
Sun et al. discloses a method for converting starting material to
olefins comprising contacting the starting material with a small
pore molecular sieve catalyst such as SAPO-34 under effective conditions
to produce olefins, wherein the molecular sieve has been modified
after synthesis by incorporation of a transition metal ion using
a transition metal compound, wherein the transition metal ion is
selected from Groups VIB, VIIB, and VII.
[0008] T. Mole, G. Bett, and D. J. Seddon, Journal of Catalysis
84 435 (1983), disclose that the presence of aromatic compounds
can accelerate the zeolite-catalyzed conversion of methanol to hydrocarbons.
The article reports ethylene yields of 5-22% when methanol is catalytically
converted in the presence of benzene or toluene over ZSM-5 at sub-atmospheric
pressure, 279.degree. to 350.degree. C., and 100% methanol conversion.
U.S. Pat. No 4499314 ('314 Patent) discloses that the addition
of various promoters, including aromatic compounds, such as toluene,
accelerate the conversion of methanol to hydrocarbons over zeolites,
such as ZSM-5 which have a pore size sufficient to permit sorption
and diffusion of the promoter. In particular, the '314 Patent teaches
that the increased conversion resulting from the addition of the
promoter allows the use of lower severity conditions, particularly
lower temperatures, which increase the yield of lower olefins (column
4 lines 17-22). Thus, in Example 1 of the patent the addition of
toluene as a promoter reduces the temperature required to achieve
full methanol conversion from 295.degree. C. to 288.degree. C. while
increasing the ethylene yield from 11 wt. % to 18 wt. %. In the
Examples of the '314 patent the methanol feedstock is diluted with
water and nitrogen such that the methanol partial pressure is less
than 2 psia (14 kPa). U.S. Pat. Nos. 4677242 and 4752651 disclose
the conversion of methanol to C.sub.2 to C.sub.4 olefins over various
silicoaluminophosphates such as SAPO-34 and "non-zeolitic
molecular sieves" (such as metal aluminophosphates), and teach
that the addition of diluents, such as aromatic materials, having
a kinetic diameter greater than the pore size of the molecular sieve,
increases the ethylene to propylene ratio in the product.
[0009] Conversion of ethane to aromatics over GaZSM-5 is disclosed
in U.S. Pat. No. 4350835. U.S. Pat. No. 4605805 discloses a
catalyst for olefin/paraffin conversion that employs ZSM-5 having
a framework wherein gallium is substituted for boron or iron. U.S.
Pat. No. 5023391 discloses a process for the direct partial oxidation
of methane with oxygen, whereby organic compounds comprising higher
hydrocarbons are produced. The catalyst used in this reaction is
a GaZSM-5 catalyst. This catalyst may be prepared by ion exchanging
or impregnating a ZSM-5 catalyst with a suitable gallium salt such
as gallium nitrate.
[0010] U.S. Pat. No. 5981817 discloses a xylene isomerization
process conducted in the presence of hydrogen and at pressures in
excess of 75 psig (620 kPa) over a ZSM-5 catalyst containing about
0.1 to 5 wt. % of at least one metal selected from the group consisting
of zinc, copper, silver and gallium. The catalyst may be a zeolite
bound zeolite prepared in accordance with U.S. Pat. No. 5460796.
[0011] All of the foregoing references are incorporated herein
by reference.
[0012] In spite of the existence of methanol conversion processes
utilizing a variety of zeolite catalysts and process conditions,
there is a continuing need to develop new procedures suitable to
convert an organic feed comprising oxygenates, such as methanol
or dimethyl ether, selectively to light olefin products and, in
particular, ethylene.
SUMMARY OF THE INVENTION
[0013] In one aspect, the present invention relates to a method
for converting oxygenates to olefins comprising contacting said
oxygenates and an aromatics co-feed with a framework gallium-containing
molecular sieve catalyst comprising pores having a size ranging
from about 5.0 Angstroms to 7.0 Angstroms, under conversion conditions
effective to produce olefins.
[0014] The method is especially useful in providing products of
relatively high ethylene/propylene ratio on a weight basis.
[0015] In another aspect, the present invention relates to a method
for converting methanol and/or dimethyl ether to a product containing
C.sub.2 and C.sub.3 olefins which comprises the step of contacting
a feed which contains methanol and/or dimethyl ether with a catalyst
comprising a framework gallium-containing ZSM-5 porous crystalline
material, said contacting step being conducted in the presence of
an aromatic compound under conversion conditions including a temperature
of 100.degree. C. to 600.degree. C. and a methanol and/or dimethyl
ether partial pressure in excess of 1 psia (6.9 kPa), and the aromatic
compound being capable of alkylation by the methanol and/or dimethyl
ether, under said conversion conditions.
[0016] In still another aspect, the present invention relates to
a catalyst composition comprising a gallium-containing ZSM-5 zeolite-bound
ZSM-5 zeolite comprising a bound ZSM-5 zeolite with framework Ga
of Si/Ga molar ratio ranging from 5 to 500 and a ZSM-5 zeolite binder
having a Si/Ga molar ratio ranging from 5 to .infin.. The catalyst
composition may comprise a component in which gallium has been introduced
by secondary synthesis or post-synthesis modification, i.e., the
catalyst composition may comprise at least one component selected
from the group consisting of bound Ga-modified ZSM-5 zeolite having
a Si/Ga molar ratio ranging from 5 to 500 and a binder of Ga-modified
ZSM-5 zeolite having a Si/Ga molar ratio ranging from 5 to .infin..
[0017] Gallium in the bound zeolite component of the catalyst is
preferably introduced to the framework of the molecular sieve during
its synthesis. Gallium can be preferably introduced into the binder
through a secondary synthesis. The catalyst of the present invention
may also be prepared using post zeolite synthesis gallium modification.
Such gallium modified materials have been subjected to ion-exchange
or wet impregnation, to the extent that such techniques provide
framework gallium in the finished catalyst. Thus, it is also possible
that some of the Ga in the catalyst may be present as non-framework
species, e.g., as a result of calcination.
DETAILED DESCRIPTION OF THE INVENTION
[0018] It is known to selectively convert oxygenates, including
methanol, to light olefins, viz., ethylene (C.sub.2.sup..dbd.) and
propylene (C.sub.3.sup..dbd.). Ethylene and propylene are in high
demand, and the need for these chemical raw materials, particularly
ethylene, continues to grow. In the present invention, oxygenate,
e.g., oxygenate selected from the group consisting of methanol and
dimethylether, is reacted at elevated temperature over a bed of
a framework gallium-containing, medium pore molecular sieve catalyst,
e.g., GaZSM-5 to produce a reaction product from which lower (C.sub.2
and C.sub.3) olefins are recovered.
Catalyst
[0019] Crystalline aluminosilicates which may be used as molecular
sieve material for the catalyst of the present invention include
intermediate pore size zeolites having an average pore size in the
range of about 5 to 7 Angstroms and a SiO.sub.2/M.sub.2O.sub.3 ratio
of at least 10 where M represents an element selected from the
group consisting of Al, B, Fe, Ga, V, and Cr.
[0020] These include zeolites having an MFI, MEL, TON, MTT or FER
crystalline structure. Preferred such zeolites include those selected
from the group consisting of ZSM-5 ZSM-11 ZSM-12 ZSM-23 ZSM-35
ZSM-48 and MCM-22 with ZSM-5 and ZSM-11 being particularly preferred.
[0021] Zeolite ZSM-5 and the conventional preparation thereof are
described in U.S. Pat. No. 3702886. Zeolite ZSM-11 and the conventional
preparation thereof are described in U.S. Pat. No. 3709979. Zeolite
ZSM-12 and the conventional preparation thereof are described in
U.S. Pat. No. 3832449. Zeolite ZSM-23 and the conventional preparation
thereof are described in U.S. Pat. No. 4076842. Zeolite ZSM-35
and the conventional preparation thereof are described in U.S. Pat.
No. 4016245. ZSM-48 and the conventional preparation thereof are
taught by U.S. Pat. No. 4375573. MCM-22 is disclosed in U.S. Pat.
No. 4954325 U.S. Pat. No. 5304698 U.S. Pat. No. 5250277
U.S. Pat. No. 5095167 and U.S. Pat. No. 5043503.
[0022] The zeolite is advantageously ion-exchanged so as to be
in its highly acidic form, e.g., HZSM-5. Where the zeolite, as synthesized,
contains alkali or alkaline earth metal cations, these can be exchanged
with ammonium cations, followed by calcination in air at 600.degree.
F. to 1000.degree. F. (316.degree. C. to 540.degree. C.) for about
1-10 hours by techniques well known in the art to produce the acid
form of the zeolite.
[0023] Silicoaluminophosphates such as SAPO-34 and "non-zeolitic
molecular sieves" (such as metal aluminophosphates) as described
in U.S. Pat. Nos. 4677242 and 4752651 can be treated with gallium
and used in the present invention.
[0024] The gallium loaded into the catalyst appears to limit the
extent of coking in the catalyst during conversion of oxygenate
to olefins, providing increased service life. The gallium may be
incorporated into the zeolite framework structure by any suitable
method of isomorphous substitution, e.g., substituting Ga atoms
for framework aluminum atoms. Alternatively, Ga can be present in
the as-synthesized zeolite by including Ga-containing materials
in the synthesis mixture, preferably before exposure of the synthesis
mixture to hydrothermal conditions. Gallium may also be introduced
inside and/or outside the framework of the molecular sieve by well
known post-synthesis gallium modification methods such as impregnation
(incipient wetness method) or by gallium ion exchange (gallium exchange).
Thus in one aspect of the present invention, the zeolite can be
impregnated with a source of gallium by well known methods such
as by contacting a solution of gallium salt dissolved in an aqueous
or alcoholic medium with the zeolite particles for a period of time
sufficient to allow the cations to penetrate the zeolite pore structure.
Suitable salts include the acetates, chlorides and nitrates. After
drying the resulting catalyst precursor, it is preferably calcined
in air at temperatures of 300.degree. F.-1000.degree. F. (149.degree.
C.-540.degree. C.) for a period of 1-24 hours. In most cases, the
metal will be present in the post-calcined zeolite structure in
the form of the metal oxide. The preferred metal loading ranges
from about 0.01 to about 10 wt. %, more preferably from about 0.1
to 2.5 wt. % metal based on the weight of the zeolite.
[0025] The gallium impregnation or exchange process described above
may be carried out before or after the porous crystalline material
is composited with the binder, but preferably before.
[0026] The porous crystalline material employed in the process
of the invention may be combined with a variety of binder or matrix
materials resistant to the temperatures and other conditions employed
in the process. Such materials include active and inactive materials
such as clays, silica and/or metal oxides such as alumina. The latter
may be either naturally occurring or in the form of gelatinous precipitates
or gels including mixtures of silica and metal oxides. Use of a
material which is active tends to change the conversion and/or selectivity
of the catalyst and hence is generally not preferred. Inactive materials
suitably serve as diluents to control the amount of conversion in
a given process so that products can be obtained in an economical
and orderly manner without employing other means for controlling
the rate of reaction. These materials may be incorporated into naturally
occurring clays, e.g., bentonite and kaolin, to improve the crush
strength of the catalyst under commercial operating conditions.
These materials, i.e., clays, oxides, etc., function as binders
for the catalyst. It is desirable to provide a catalyst having good
crush strength because in commercial use it is desirable to prevent
the catalyst from breaking down into powder-like materials. These
clay and/or oxide binders have been employed normally only for the
purpose of improving the crush strength of the catalyst. Naturally
occurring clays which can be composited with the porous crystalline
material include the montmorillonite and kaolin family, which families
include the subbentonites, and the kaolins commonly known as Dixie,
McNamee, Georgia and Florida clays or others in which the main mineral
constituent is halloysite, kaolinite, dickite, nacrite, or anauxite.
Such clays can be used in the raw state as originally mined or initially
subjected to calcination, acid treatment or chemical modification.
In addition to the foregoing materials, the porous crystalline material
can be composited with a porous matrix material such as silica-alumina,
silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia,
silica-titania as well as ternary compositions such as silica-alumina-thoria,
silica-alumina-zirconia, silica-alumina-magnesia and silica-magnesia-zirconia.
[0027] The relative proportions of porous crystalline material
and inorganic oxide matrix vary widely, with the content of the
former ranging from about 1 to about 90% by weight and more usually,
particularly when the composite is prepared in the form of beads,
in the range of about 2 to about 80 wt. % of the composite. Preferably,
the binder material comprises silica or a kaolin clay. Procedures
for preparing silica-bound zeolites, such as ZSM-5 are described
in U.S. Pat. Nos. 4582815; 5053374; and 5182242. A particular
procedure for binding ZSM-5 with a silica binder involves an extrusion
process. The porous crystalline material may be combined with a
binder in the form of a fluidized bed catalyst. This fluidized bed
catalyst may comprise clay in the binder thereof, and may be formed
by a spray-drying process to form catalyst particles having a particle
size of 20-200 microns.
[0028] In one aspect of the present invention, porous crystalline
materials comprise a zeolite in the form of zeolite-bound particles
such as those prepared in accordance with U.S. Pat. No. 5460796
the disclosure of which is incorporated herein by reference. In
such cases where the catalyst is a zeolite bound zeolite (ZBZ),
it can have a bound zeolite of Si/Ga molar ratio ranging from 5
to 500 preferably from 20 to 200 most preferably from 30 to 100
and a binder having a Si/Ga molar ratio ranging from 5 to .infin.,
preferably from 20 to .infin., most preferably from 100 to .infin..
In one embodiment, the catalyst can be a zeolite bound zeolite having
a bound zeolite of framework Ga-containing zeolite having a Si/Ga
molar ratio ranging from 5 to 500 more preferably from 20 to 200
and most preferably from 30 to 100 and a binder of Ga-exchanged
and/or Ga-impregnated zeolite having a Si/Ga molar ratio ranging
from 5 to .infin., more preferably from 20 to .infin. and most preferably
from 100 to .infin..
[0029] The catalyst employed of the invention preferably has a
very low acid activity. Using the alpha test of acid activity disclosed
in Journal of Catalysis, volume 61 page 395 (1980), the entire
disclosure of which is incorporated by reference herein, the catalyst
of the invention preferably has an alpha value less than 10 more
preferably less than 5 and most preferably less than 2.
Feed
[0030] The feed employed in the present invention comprises an
oxygenate component. Such oxygenates can be selected from the group
consisting of methanol, ethanol, n-propanol, isopropanol, C.sub.4-C.sub.20alcohols,
methyl ethyl ether, di-methyl ether, di-ethyl ether, di-isopropyl
ether, methyl isopropyl ether, ethyl isopropyl ether, di-methyl
carbonate, carbonyl compounds, and mixtures thereof. Preferred oxygenates
are selected from the group consisting of methanol and dimethyl
ether, with methanol being especially preferred.
[0031] An aromatics co-feed is added to the feed of the present
invention. Non-limiting examples of such aromatics which comprise
aromatic compounds which can diffuse into channels or cages of said
catalyst together with oxygenate, can be selected from the group
consisting of benzene, toluene, xylenes, light reformates, full-range
reformates or any distilled fraction thereof, coker naphtha or any
distilled fraction thereof, FCC naphtha or any distilled fraction
thereof, steam crack naphtha or any distilled fraction thereof and
coal derived aromatics.
[0032] Aromatics co-feed selected from the group of aromatic compounds
consisting of toluene and xylenes is especially preferred. In one
embodiment of the invention, the aromatics co-feed can be added
to the feed upstream of the reactor.
[0033] Minor amounts, say, 0.01-10 wt. %, of aromatics such as
benzene, toluene, and/or xylenes, etc. can be co-fed with the oxygenate
in order to enhance olefin selectivity. The aromatics component
can be added relative to the oxygenate, e.g., methanol and/or dimethyl
ether, in an amount sufficient to provide a feed having a molar
ratio of oxygenate to aromatic compound greater than 0.1:1 and less
than 300:1 preferably greater than 2:1 and less than 150:1 and
more preferably greater than 5:1 and less than 50:1.
[0034] In one aspect of the invention, the co-fed aromatic may
consist only of the xylene isomers, or may be a mixture of the xylene
isomers with another aromatic hydrocarbon such as ethylbenzene,
benzene, toluene, ethyltoluene, trimethylbenzene, diethylbenzene,
ethylxylene, and tetramethylbenzene. In the latter case, the xylene
isomeric mixture is present desirably in an amount of generally
at least 30% by weight, preferably at least 50% by weight, based
on the weight of the aromatic hydrocarbon feed.
[0035] C.sub.8 aromatic hydrocarbon fractions obtained by reforming,
thermal cracking or hydrocracking of naphtha can be used as the
aromatic hydrocarbon additive to the oxygenate feed in the process
of this invention. These fractions contain ethylbenzene of the same
number of carbons in addition to the xylene isomers. Very good results
can be obtained in the process of this invention when using a C.sub.8-aromatic
hydrocarbon fraction which contains the xylene isomers and ethylbenzene
in a total amount of at least 80%.
[0036] The present method can be conducted in the presence of added
diluent, e.g., hydrogen and/or added water such that the molar ratio
of diluent to oxygenate in the feed to the reactor is between about
0.01 and about 10 preferably between about 0.01 and 5 and more
preferably between about 0.01 and 2. Those skilled in the art will
be capable of adjusting the various reaction parameters and conditions
to optimize conversion, yield, and selectivity, using routine experimentation.
Reaction Conditions
[0037] The process of the invention is preferably carried out in
a moving or fluid catalyst bed with continuous oxidative regeneration.
The extent of coke loading can then be continuously controlled by
varying the severity and/or the frequency of regeneration. The conversion
of oxygenate to olefin according to the present invention may occur
in a reactor of any configuration. Continuous reactors, such as
dense fluidized bed, riser, fast fluid-bed, or fixed bed, are suitable
configurations for use in the present invention. The process of
the invention is preferably carried out in a moving or fluid catalyst
bed with continuous oxidative regeneration. The extent of coke loading
can then be continuously controlled by varying the severity and/or
the frequency of regeneration.
[0038] Preferably, the reactor is a fluidized bed flow reactor
type. The catalyst may be used in various forms, such as a fixed
bed, moving bed, fluidized bed, e.g., a dense fluid bed, in suspension
in the generally gaseous reaction mixture.
[0039] The fluidized beds used in the present invention can be
relatively dense, such as turbulent sub-transport fluid beds with
an operating bed density of about 200 to 700 kg/m.sup.3 preferably
about 300 to 500 kg/m.sup.3. The use of these dense beds increases
the catalyst concentration at the area of oxygenate injection.
[0040] A suitable alternate fluid bed design is termed a fast fluid
bed. This is typically characterized by a bed density that is lower
than that prevailing in a dense fluid bed. Superficial gas velocities
are typically greater than 5 feet/second (1.5 meters/second). The
fluidized bed in a fast fluid bed is less defined than that in a
dense fluid bed.
[0041] Another suitable reactor system is a riser "fluid bed"
reactor. In this case the classical fluidized bed does not exist
but rather solid catalyst particles and gas are flowed up or down
the reactor vessel in a more or less homogeneous manner. Typically
solids density in the riser is less than about 100 kg/m.sup.3 and
the superficial gas velocity is in excess of 20-40 feet/second (6-12
meters/second).
[0042] Liquids can be injected into all three types of fluid bed
systems mentioned above.
[0043] The catalyst inventory of the fluidized beds can be maintained
by return of solids to the beds from the cyclone recovery system,
but small losses will occur, e.g., due to attrition. Losses can
be made up by adding catalyst to maintain catalyst inventory.
[0044] The production conditions of the present invention can comprise
a temperature of from about 100.degree. C. to about 600.degree.
C., a pressure of from about 1 psia to 200 psia (6.9 kPa to 1380
kPa), and a weight hourly space velocity in the range of from about
0.01 to about 500 hr.sup.-1 preferably a temperature of 350.degree.
C. to 480.degree. C., a pressure of from about 5 psia to 100 psia
(34 kPa to 680 kPa), and a weight hourly space velocity in the range
of from about 2 to about 100 hr.sup.-1.
[0045] These conditions can be effective to provide an ethylene/propylene
product weight ratio ranging from 0.1 to 7 preferably at least
1. Combinations of temperatures of 250.degree. to 480.degree. C.
and ethylene/propylene product weight ratio ranging from 0.1 to
7 and preferably 300.degree. to 450.degree. C. and ethylene/propylene
product weight ratio of at least 1 can be used.
[0046] Operating at higher oxygenate, e.g., methanol and/or dimethylether,
partial pressures can allow the absolute yield per pass of olefin
product to be increased. A suitable methanol and/or dimethylether
partial pressure for use in the process of the invention is in excess
of 10 psia (70 kPa), preferably 15 to 150 psia (103 to 1030 kPa).
[0047] The present invention can employ oxygenate conversion levels
of greater than 95%, greater than 98% or even greater than 99%,
e.g., 100%.
[0048] Suitable control of the oxygenate, e.g., methanol, conversion
can, of course, be achieved by variation of the weight hourly space
velocity, which typically can vary between about 0.1 and 100 preferably
between about 0.1 and 10. The process of the invention can convert
oxygenate, e.g., methanol and/or dimethyl ether to a light olefin-containing
stream with ethylene selectivity of at least 25%, preferably at
least 40%, and propylene selectivity of at least 30%, preferably
at least 40% as well as an ethylene/propylene ratio of greater than
0.6 preferably greater than about 1. Such high selectivities for
lower olefins can be achieved even at high oxygenate conversions,
e.g., greater than 99%.
[0049] The following examples are provided to more fully illustrate
the invention and accent its advantageous features. These examples
are included to illustrate the invention and should not be construed
as limiting it in any way.
EXAMPLE 1
Catalyst Preparation
[0050] A gallium-substituted zeolite bound zeolite, ZSM-5 i.e.,
H(Ga)ZSM-5 was prepared in accordance with the details provided
below:
[0051] 1. Preparation of the Crystals to be Bound Having a SiO.sub.2/G.sub.2O.sub.3
Ratio of 90 and a Crystal Size of Approximately 3 microns:
[0052] This was carried out by a procedure similar to Example 1
of U.S. Pat. No. 6040259 incorporated herein by reference. The
synthesis mixture consisted of the following parts (by weight):
[0053] Part A: NaOH, Ga.sub.2O.sub.3 (99.999 wt. %) and water were
used to make a solution by boiling the ingredients and cooling to
room temperature, about 25.degree. C. Water lost during boiling
was replaced.
[0054] Part B: Colloidal silica in water.
[0055] Part C: Tetrapropylammonium Bromide (TPABr) and water.
[0056] Part D: MFI (ZSM-5) seeds.
[0057] Part B was added to Part C and mixed. Then Part D was added.
Finally Part A was added and mixed until the resulting mixture was
homogeneous. The final reaction mixture had the following composition:
[0058] 0.45 Na.sub.2O/0.9 TPABr/0.111 Ga.sub.2O.sub.3/10 SiO.sub.2/147
H.sub.2O and 1.4 wt ppm of MFI seeds. The final reaction mixture
was heated to 140.degree. C. for about 24 hrs and the resulting
powder was washed, dried and calcined.
[0059] 2. Preparation of Silica Bound Crystals in a Ratio of Bound
Zeolite/binder of 70/30:
[0060] A mixture was made and extruded according to U.S. Pat. No.
5460796 incorporated herein by reference, Example 1 Step B.
The resulting extrudates were calcined.
[0061] 3. Preparation of GaMFI Bound GaMFI (Secondary Synthesis):
[0062] Part A: calcined silica bound GaMFI prepared under Step
2 above.
[0063] Part B: solution containing NaOH, Ga.sub.2O.sub.3 TPABr
and water.
[0064] Parts A and B were combined and the resulting mixture had
the following overall composition: 0.48 Na.sub.2O/0.7 TPABr/0.05
Ga.sub.2O.sub.3/10 SiO.sub.2/149 H.sub.2O exclusive of the GaMFI
crystals.
[0065] The 10 moles of SiO.sub.2 are the binder of the extrudates.
The mixture is heated at 150.degree. C. for 80 hrs to convert the
binder to GaMFI (GaZSM-5).
[0066] 4. Calcination and Ion Exchange:
[0067] The resulting extrudates of Step 3 were washed, dried, and
calcined. Then the extrudates were ion-exchanged with NH.sub.4NO.sub.3
and calcined to obtain the final catalyst.
[0068] The bound zeolite of the catalyst had a Si/Ga ratio of about
45 while that of the binder was about 100. The catalyst was calcined
at 450.degree. C. in air overnight prior to use.
Light Olefins Production
[0069] The production of light olefins was carried out by mixing
50.0 mg of a selected Ga-containing catalyst prepared following
steps 1-4 in Example 1 with 1.0 gram of silicon carbide. The resulting
mixture was then placed in a 0.50 inch (1.3 cm) (outside diameter)
No. 304 stainless steel tubular reactor having a wall thickness
of 0.063 inch (0.16 cm). A thermocouple was provided in direct contact
with the catalyst for temperature measurements. The catalyst was
activated in situ by heating in flowing helium for one hour at 450.degree.
C. in the reactor.
[0070] Feedstock comprising methanol and aromatics co-feed was
introduced to the tubular reactor by means of a Valco four-port
injection valve with a fixed volume of 3 l concurrently with a stream
of helium diluent. The pressure and temperature employed in this
Example were 40 psia (275 kPa) and 450.degree. C., respectively.
The effluent from the tubular reactor following each pulse of methanol
was collected using a Valco six-port valve with a 2 ml sample loop.
The collected effluent sample was analyzed by gas chromatography
(Hewlett Packard 6890) equipped with a flame ionization detector.
The chromatographic column used was a 150 meter, 0.25 mm (i.d.)
fused silica capillary column (Model No. Petrocol DH 150).
[0071] In calculating selectivities reported in Table 2 to Table
7 dimethyl ether is not counted as one of the converted products
from methanol, i.e., dimethyl ether is treated as if it were methanol.
Nor are toluene, xylenes and other alkylated aromatics that are
produced from methanol counted as converted products because these
aromatics are recycled.
[0072] Tables 1-6 below set out product distributions (wt. %) and
ethylene to propylene molar ratios for methanol and methanol/aromatic
feeds of Examples 2-7.
EXAMPLE 2 (Comparative)
[0073] Pure methanol feed was reacted on H(Ga)ZSM-5 from Example
1 as a control for a para-xylene co-feed experiment at 100 wt. %
MeOH conversion. The product distributions (wt. %) and ethylene
to propylene molar ratio are listed in Table 1 below. C.sub.3.sup.0
and C.sub.4.sup.0 refer to propane and butane while C.sub.2.dbd.
and C.sub.3.dbd. refer to ethylene and propylene.
1TABLE 1 CH.sub.4 C.sub.2= C.sub.3= C.sub.3.sup.0 C.sub.4.sup.0
C.sub.4= C.sub.5= C.sub.2=/C.sub.3= 0.5 5.9 49.8 0.3 1.1 24.0 12.5
0.12
EXAMPLE 3
[0074] A mixture of 50/50 wt. % of methanol and para-xylene is
reacted in the presence of H(Ga) ZSM-5 under the same conditions
as in Comparative Example 2. The results set out in Table 2 below
show an increase in the total ethylene and propylene selectivity
of 77.5 wt. % compared to Example 2's 55.7 wt. %).
[0075] The selectivity to ethylene is higher than that of propylene,
in contrast to Comparative Example 2 where the selectivity to ethylene
was negligible.
2TABLE 2 CH.sub.4 C.sub.2= C.sub.3= C.sub.3.sup.0 C.sub.4.sup.0
C.sub.4= C.sub.5= C.sub.2=/C.sub.3= 3.8 44.0 33.5 1.5 0.5 11.1 5.6
1.31
EXAMPLE 4
[0076] A mixture of 90/10 wt. % of methanol and para-xylene is
reacted in the presence of H(Ga) ZSM-5 under the same conditions
as in Example 2. The results in Table 3 show a high selectivity
to ethylene and propylene (65.7 wt. %) is maintained. Although the
ethylene to propylene ratio is lower than that in Example 3 in
some embodiments such lower ratio is preferred. Despite ethylene's
higher value as a commodity than propylene, there are occasions
where high propylene selectivity is desired. This experiment shows
that ethylene to propylene ratio can be varied by simply adjusting
the amount of aromatics in the feed.
3TABLE 3 CH.sub.4 C.sub.2= C.sub.3= C.sub.3.sup.0 C.sub.4.sup.0
C.sub.4= C.sub.5= C.sub.2=/C.sub.3= 1.7 21.4 44.3 1.6 1.0 19.5 10.4
0.48
EXAMPLE 5
[0077] A mixture of 10/90 wt. % of methanol/para-xylene was reacted
on the H(Ga)ZSM-5 catalyst of Example 1 under the same conditions
as in Example 2. The results in Table 4 show high selectivity to
ethylene and propylene (89.8 wt. %) with very high ethylene selectivity
(72.7 wt. %).
4TABLE 4 CH.sub.4 C.sub.2= C.sub.3= C.sub.3.sup.0 C.sub.4.sup.0
C.sub.4= C.sub.5= C.sub.2=/C.sub.3= 3.1 72.7 17.1 0.5 0.0 4.3 2.3
4.25
EXAMPLE 6 (Comparative)
[0078] The performance of an HZSM-5 catalyst with a SiO.sub.2/Al.sub.2O.sub.3
ratio of 218 prepared in accordance with Example 1A of U.S. Pat.
No. 5460796 was studied in the pulse reactor for the co-feeding
experiment using a 50/50 wt. % mixture of methanol/p-xylene at 64
wt. % methanol and dimethyl ether (DME) conversion. The results
shown in Table 5 below indicate a significant decrease in total
ethylene and propylene selectivity (70.3 wt. %) at 64 wt. % conversion
compared with H(Ga)ZSM-5 in Example 3 (77.5 wt. %) at 100 wt. %
conversion. C.sub.2.dbd./C.sub.3.dbd. product ratio is also improved
for the present invention (1.31 vs. 0.77).
5TABLE 5 CH.sub.4 C.sub.2= C.sub.3= C.sub.3.sup.0 C.sub.4.sup.0
C.sub.4= C.sub.5= C.sub.2=/C.sub.3= 2.5 30.5 39.8 0.5 0.0 16.0 10.7
0.77
EXAMPLE 7
[0079] The effect of temperature on olefin selectivities was studied
by co-feeding toluene with methanol on H(Ga)ZSM-5 as prepared in
Example 1 at 100 wt. % methanol conversion. The results are summarized
in Table 6 below.
[0080] The reaction temperature has a significant effect on olefin
selectivity. As temperature is increased, the ethylene and propylene
selectivity increases as well, inasmuch as the thermodynamics favor
lighter products. However, at higher temperatures methane selectivity
increased significantly. For example, the selectivity for methane
was as high as 33.2 wt. % at 550.degree. C. Accordingly, it is preferred
that the temperature range be in the range of 100 to 600.degree.
C., preferably in the range of 250 to 480.degree. C., more preferably
in the range of 300 to 450.degree. C., where methane make is to
be minimized.
[0081] While the invention has been described herein in terms of
various preferred embodiments, those skilled in the art will recognize
that various changes and modifications can be made without departing
from the spirit and scope of the invention, as defined in the following
claims. |