Molecular sieve abstract
A catalyst for converting methanol to light olefins along with
the process itself are disclosed and claimed. The catalyst is a
metalloaluminophosphate molecular sieve having the empirical formula
(EL.sub.x Al.sub.y P.sub.z)O.sub.2 where EL is a metal such as silicon
or magnesium and x, y and z are the mole fractions of EL, Al and
P respectively. The molecular sieve has predominantly a plate crystal
morphology in which the average smallest crystal dimension is at
least 0.1 microns and has an aspect ratio of no greater than 5.
Use of this catalyst gives a product with a larger amount of ethylene
versus propylene.
Molecular sieve claims
We claim as our invention:
1. A process for converting methanol to light olefins comprising
contacting the methanol with a catalyst at conversion conditions,
the catalyst comprising a crystalline metallo aluminophosphate molecular
sieve having a chemical composition on an anhydrous basis expressed
by an empirical formula of:
where EL is a metal selected from the group consisting of silicon,
magnesium, zinc, iron, cobalt, nickel, manganese, chromium and mixtures
thereof, "x" is the mole fraction of EL and has a value
of at least 0.005 "y" is the mole fraction of Al and
has a value of at least 0.01 "z" is the mole fraction
of P and has a value of at least 0.01 and x+y+z=1 the molecular
sieve characterized in that it has predominantly a plate crystal
morphology, wherein the average smallest crystal dimension is at
least 0.1 micron and has an aspect ratio of less than or equal to
5.
2. The process of claim 1 where the EL metal is selected from the
group consisting of silicon, magnesium, cobalt and mixtures thereof.
3. The process of claim 2 where EL is silicon.
4. The process of claim 3 where the silicon aluminophosphate has
the crystal structure of SAPO-34.
5. The process of claim 1 where the catalyst comprises a metallo-alumino-phosphate
molecular sieve and an inorganic oxide binder.
6. The process of claim 5 where the binder is selected from the
group consisting of alumina, silica, aluminum phosphate, silica-alumina
and mixtures thereof.
7. The process of claim 5 where the molecular sieve is present
in an amount from about 10 to about 90 weight percent of the catalyst.
8. The process of claim 7 where the molecular sieve is present
in an amount from about 30 to about 70 weight percent of the catalyst.
9. The process of claim 1 where the conversion conditions are a
temperature of about 300.degree. C. to about 600.degree. C., a pressure
of about 0 kPa to about 17224 kPa (250 psig) and a weight hourly
space velocity of about 1 to about 100 hr.sup.-1.
10. The process of claim 1 where the average smallest crystal dimension
is at least 0.2 microns.
11. The process of claim 1 where the metal aluminophosphate has
a metal (EL) content from about 0.005 to about 0.05 mole fraction.
12. The process of claim 1 where the aspect ratio is less than
or equal to 2 and the crystal morphology is cubic.
13. A catalyst for converting methanol to light olefins comprising
a crystalline metallo aluminophosphate molecular sieve having an
empirical chemical composition on an anhydrous basis expressed by
the formula:
where EL is a metal selected from the group consisting of silicon,
magnesium, zinc, iron, cobalt, nickel, manganese, chromium and mixtures
thereof, "x" is the mole fraction of EL and has a value
of at least 0.005 "y" is the mole fraction of Al and
has a value of at least 0.01 "z" is the mole fraction
of P and has a value of at least 0.01 and x+y+z=1 the molecular
sieve characterized in that it has predominantly a plate crystal
morphology, wherein the average smallest crystal dimension is at
least 0.1 micron and has an aspect ratio no greater than 5.
14. The catalyst of claim 13 where the EL metal is selected from
the group consisting of silicon, magnesium, cobalt and mixtures
thereof.
15. The catalyst of claim 14 where the EL metal is silicon.
16. The catalyst of claim 15 where the silicon aluminophosphate
has the crystal structure of SAPO-34.
17. The catalyst of claim 13 where the catalyst comprises a metal
aluminophosphate molecular sieve and an inorganic oxide binder.
18. The catalyst of claim 17 where the binder is selected from
the group consisting of alumina, silica, aluminum phosphate, silica-alumina
and mixtures thereof.
19. The catalyst of claim 13 where the molecular sieve is present
in an amount from about 10 to about 90 weight percent of the catalyst.
20. The catalyst of claim 19 where the molecular sieve is present
in an amount from about 30 to about 70 weight percent of the catalyst.
21. The catalyst of claim 13 where the average smallest crystal
dimension is at least 0.2 microns.
22. The catalyst of claim 13 where the aluminophosphate has a metal
(EL) content from about 0.005 to about 0.05 mole fraction.
23. The catalyst of claim 13 where the aspect ratio is less than
or equal to 2 and the crystal morphology is cubic.
Molecular sieve description
FIELD OF THE INVENTION
This invention relates to a process for converting methanol to
light olefins and to a catalyst for carrying out the process. The
catalyst comprises a metallo aluminophosphate molecular sieve having
an empirical formula of (EL.sub.x Al.sub.y P.sub.z)O.sub.2 where
EL includes silicon and characterized in that the molecular sieve
has predominantly a plate crystal morphology such that the average
smallest crystal dimension is at least 0.1 micron and has an aspect
ratio of less than or equal to 5.
BACKGROUND OF THE INVENTION
The limited supply and increasing cost of crude oil has prompted
the search for alternative processes for producing hydrocarbon products.
One such process is the conversion of methanol to hydrocarbons and
especially light olefins (by light olefins is meant C.sub.2 to C.sub.4
olefins). The interest in the methanol to olefin (MTO) process is
based on the fact that methanol can be obtained from coal or natural
gas by the production of synthesis gas which is then processed to
produce methanol.
Processes for converting methanol to light olefins are well known
in the art. Initially aluminosilicates or zeolites were used as
the catalysts necessary to carry out the conversion. For example,
see U.S. Pat. Nos. 4238631; 4328384 4423274. These patents
further disclose the deposition of coke onto the zeolites in order
to increase selectivity to light olefins and minimize the formation
of C.sub.5 + byproducts. The effect of the coke is to reduce the
pore diameter of the zeolite.
The prior art also discloses that silico aluminophosphates (SAPOs)
can be used to catalyze the methanol to olefin process. Thus, U.S.
Pat. No. 4499327 discloses that many of the SAPO family of molecular
sieves can be used to convert methanol to olefins. The '327 patent
also discloses that preferred SAPOs are those that have pores large
enough to adsorb xenon (kinetic diameter of 4.0 .ANG.) but small
enough to exclude isobutane (kinetic diameter of 5.0 .ANG.). A particularly
preferred SAPO is SAPO-34.
U.S. Pat. No. 4752651 discloses the use of nonzeolitic molecular
sieves (NZMS) including ELAPOs and MeAPO molecular sieves to catalyze
the methanol to olefin reaction.
The effect of the particle size of the molecular sieve on activity
has also been documented in U.S. Pat. No. 5126308. In the '308
patent it is disclosed that molecular sieves in which 50% of the
molecular sieve particles have a particle size less than 1.0 .mu.m
and no more than 10% of the particles have a particle size greater
than 2.0 .mu.m have increased activity and/or durability. The '308
patent also discloses that restricting the silicon content to about
0.005 to about 0.05 mole fraction also improves catalytic performance.
In contrast to this art, applicants have found that molecular sieves
having the empirical formula (EL.sub.x Al.sub.y P.sub.z)O.sub.2
(hereinafter ELAPO) where EL is a metal selected from the group
consisting of silicon, magnesium, zinc, iron, cobalt, nickel, manganese,
chromium and mixtures thereof and x, y and z are the mole fractions
of EL, Al and P respectively and having a predominantly plate crystal
morphology wherein the average smallest crystal dimension is at
least 0.1 micron and has an aspect ratio of less than or equal to
5. These molecular sieves produce a higher amount of ethylene versus
propylene. This increased selectivity is a very desirable feature
of a MTO catalyst. This morphology is obtained by controlling the
metal (EL) content of the molecular sieve and the crystallization
time during synthesis of the molecular sieve.
SUMMARY OF THE INVENTION
As stated, this invention relates to an ELAPO containing catalyst
and a process for converting methanol to light olefins using the
catalyst. Accordingly, one embodiment of the invention is a process
for converting methanol to light olefins comprising contacting the
methanol with a catalyst at conversion conditions, the catalyst
comprising a crystalline metal aluminophosphate molecular sieve
having a chemical composition on an anhydrous basis expressed by
an empirical formula of:
where EL is a metal selected from the group consisting of silicon,
magnesium, zinc, iron, cobalt, nickel, manganese, chromium and mixtures
thereof, "x" is the mole fraction of EL and has a value
of at least 0.005 "y" is the mole fraction of Al and
has a value of at least 0.01 "z" is the mole fraction
of P and has a value of at least 0.01 and x+y+z=1 the molecular
sieve characterized in that it has predominantly a plate crystal
morphology, wherein the average smallest crystal dimension is at
least 0.1 micron and has an aspect of less than or equal to 5.
Another embodiment of the invention is a catalyst for converting
methanol to light olefins comprising a crystalline metallo aluminophosphate
molecular sieve having an empirical chemical composition on an anhydrous
basis expressed by the formula:
where EL is a metal selected from the group consisting of silicon,
magnesium, zinc, iron, cobalt, nickel, manganese, chromium and mixtures
thereof, "x" is the mole fraction of EL and has a value
of at least 0.005 "y" is the mole fraction of Al and
has a value of at least 0.01 "z" is the mole fraction
of P and has a value of at least 0.01 and x+y+z=1 the molecular
sieve characterized in that it has a crystal morphology wherein
the average smallest crystal dimension is at least 0.1 micron.
These and other objects and embodiments of the invention will become
more apparent after the detailed description of the invention.
DETAILED DESCRIPTION OF THE INVENTION
An essential feature of the process of the instant invention is
an ELAPO molecular sieve. ELAPOs are molecular sieves which have
a three-dimensional microporous framework structure of AlO.sub.2
PO.sub.2 and ELO.sub.2 tetrahedral units. Generally the ELAPOs have
the empirical formula
(EL.sub.x Al.sub.y P.sub.z)O.sub.2
where EL is a metal selected from the group consisting of silicon,
magnesium, zinc, iron, cobalt, nickel, manganese, chromium and mixtures
thereof, "x" is the mole fraction of EL and has a value
of at least 0.005 "y" is the mole fraction of Al and
has a value of at least 0.01 "z" is the mole fraction
of P and has a value of at least 0.01 and x+y+z=1. When EL is a
mixture of metals, "x" represents the total amount of
the metal mixture present. Preferred metals (EL) are silicon, magnesium
and cobalt with silicon being especially preferred.
The preparation of various ELAPOs are well known in the art and
may be found in U.S. Pat. No.: 4554143 (FeAPO); U.S. Pat. No.
4440871 (SAPO); U.S. Pat. No. 4853197 (MAPO, MnAPO, ZnAPO, CoAPO);
U.S. Pat. No. 4793984 (CAPO), U.S. Pat. No. 4752651 and U.S.
Pat. No. 4310440 all of which are incorporated by reference.
Generally, the ELAPO molecular sieves are synthesized by hydrothermal
crystallization from a reaction mixture containing reactive sources
of EL, aluminum, phosphorus and a templating agent. Reactive sources
of EL are the metal salts such as the chloride and nitrate salts.
When EL is silicon a preferred source is fumed, colloidal or precipitated
silica. Preferred reactive sources of aluminum and phosphorus are
pseudo-boehmite alumina and phosphoric acid. Preferred templating
agents are amines and quaternary ammonium compounds. An especially
preferred templating agent is tetraethylammonium hydroxide (TEAOH).
The reaction mixture is placed in a sealed pressure vessel, optionally
lined with an inert plastic material such as polytetrafluoroethylene
and heated preferably under autogenous pressure at a temperature
between about 50.degree. C. and 250.degree. C. and preferably between
about 100.degree. C. and 200.degree. C. for a time sufficient to
produce crystals of the ELAPO molecular sieve. Typically the time
varies from about 1 hour to about 120 hours and preferably from
about 24 hours to about 48 hours. The desired product is recovered
by any convenient method such as centrifugation or filtration.
The ELAPO molecular sieves of this invention have predominantly
a plate crystal morphology. By predominantly is meant greater than
50% of the crystals. Preferably at least 70% of the crystals have
a plate morphology and most preferably at least 90% of the crystals
have a plate morphology. Especially good selectivity (C.sub.2.sup.=
versus C.sub.3.sup.=) is obtained when at least 95% of the crystals
have a plate morphology. By plate morphology is meant that the crystals
have the appearance of rectangular slabs. More importantly, the
aspect ratio is less than or equal to 5. The aspect ratio is defined
as the ratio of the largest crystalline dimension divided by the
smallest crystalline dimension. A preferred morphology which is
encompassed within the definition of plate is cubic morphology.
By cubic is meant not only crystals in which all the dimensions
are the same, but also those in which the aspect ratio is less than
or equal to 2. It is also necessary that the average smallest crystal
dimension be at least 0.1 microns and preferably at lest 0.2 microns.
As is shown in the examples, the morphology of the crystals and
the average smallest crystal dimension is determined by examining
the ELAPO molecular sieve using Scanning Electron Microscopy (SEM)
and measuring the crystals in order to obtain an average value for
the smallest dimension.
Without wishing to be bound by any one particular theory, it appears
that a minimum thickness is required so that the diffusion path
for the desorption of ethylene and propylene is sufficiently long
to allow differentiation of the two molecules. Since ethylene is
a more valuable product, by controlling the crystal dimensions one
can maximize the formation of ethylene. As will be shown in the
examples, when the smallest dimension is less than 0.1 the ratio
of ethylene to propylene (C.sub.2.sup.= /C.sub.3.sup.=) is about
1.2 whereas when the smallest dimension is greater than 0.1 microns,
the ratio of C.sub.2.sup.32 /C.sub.3.sup.= is about 1.4. This provides
a greater production of ethylene.
The ELAPOs which are synthesized using the process described above
will usually contain some of the organic templating agent in its
pores. In order for the ELAPOs to be active catalysts, the templating
agent in the pores must be removed by heating the ELAPO powder in
an oxygen containing atmosphere at a temperature of about 200.degree.
to about 700.degree. C. until the template is removed, usually a
few hours.
A preferred embodiment of the invention is one in which the metal
(EL) content varies from about 0.005 to about 0.05 mole fraction.
If EL is more than one metal then the total concentration of all
the metals is between about 0.005 and 0.05 mole fraction. An especially
preferred embodiment is one in which EL is silicon (usually referred
to as SAPO). The SAPOs which can be used in the instant invention
are any of those described in U.S. Pat. No. 4440871. Of the specific
crystallographic structures described in the '871 patent, the SAPO-34
i.e., structure type 34 is preferred. The SAPO-34 structure is
characterized in that it adsorbs zenon but does not adsorb isobutane,
indicating that it has a pore opening of about 4.2 .ANG..
The ELAPO molecular sieve of this invention may be used alone or
they may be mixed with a binder and formed into shapes such as extrudates,
pills, spheres, etc. Any inorganic oxide well known in the art may
be used as a binder. Examples of the binders which can be used include
alumina, silica, aluminum-phosphate, silica-alumina, etc. When a
binder is used, the amount of ELAPO which is contained in the final
product ranges from 10 to 90 weight percent and preferably from
30 to 70 weight percent.
The conversion of methanol to light olefins is effected by contacting
the methanol with the ELAPO catalyst at conversion conditions, thereby
forming the desired light olefins. The methanol can be in the liquid
or vapor phase with the vapor phase being preferred. Contacting
the methanol with the ELAPO catalyst can be done in a continuous
mode or a batch mode with a continuous mode being preferred. The
amount of time that the methanol is in contact with the ELAPO catalyst
must be sufficient to convert the methanol to the desired light
olefin products. When the process is carried out in a batch process,
the contact time varies from about 0.001 hr. to about 1 hr. and
preferably from about 0.01 hr. to about 1.0 hr. The longer contact
times are used at lower temperatures while shorter times are used
at higher temperatures. Further, when the process is carried out
in a continuous mode, the Weight Hourly Space Velocity (WHSV) based
on methanol can vary from about 1 hr.sup.-1 to about 1000 hr.sup.-1
and preferably from about 1 hr.sup.-1 to about 100 hr.sup.-1.
Generally, the process must be carried out at elevated temperatures
in order to form light olefins at a fast enough rate. Thus, the
process should be carried out at a temperature of about 300.degree.
C. to about 600.degree. C., preferably from about 400.degree. C.
to about 550.degree. C. and most preferably from about 450.degree.
C. to about 525.degree. C. The process may be carried out over a
wide range of pressure including autogenous pressure. Thus, the
pressure can vary from about 0 kPa (0 psig) to about 1724 kPa (250
psig) and preferably from about 34 kPa (5 psig) to about 345 kPa
(50 psig).
Optionally, the methanol feedstock may be diluted with an inert
diluent in order to more efficiently convert the methanol to olefins.
Examples of the diluents which may be used are helium, argon, nitrogen,
carbon monoxide, carbon dioxide, hydrogen, steam, paraffinic hydrocarbons,
e.g., methane, aromatic hydrocarbons, e.g., benzene, toluene and
mixtures thereof. The amount of diluent used can vary considerably
and is usually from about 5 to about 90 mole percent of the feedstock
and preferably from about 25 to about 75 mole percent.
The actual configuration of the reaction zone may be any well known
catalyst reaction apparatus known in the art. Thus, a single reaction
zone or a number of zones arranged in series or parallel may be
used. In such reaction zones the methanol feedstock is flowed through
a bed containing the ELAPO catalyst. When multiple reaction zones
are used, one or more ELAPO catalyst may be used in series to produce
the desired product mixture. Instead of a fixed bed, a dynamic bed
system, e.g., fluidized or moving, may be used. Such a dynamic system
would facilitate any regeneration of the ELAPO catalyst that may
be required. If regeneration is required, the ELAPO catalyst can
be continuously introduced as a moving bed to a regeneration zone
where it can be regenerated by means such as oxidation in an oxygen
containing atmosphere to remove carbonaceous materials.
The following examples are presented in illustration of this invention
and are not intended as undue limitations on the generally broad
scope of the invention as set out in the appended claims |