Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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"METALLO ALUMINOPHOSPHATE MOLECULAR
SIEVE WITH CUBIC CRYSTAL MORPHOLOGY
AND METHANOL TO OLEFIN PROCESS USING THE SIEVE"
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 C2 to C4 olefins). The interest in the methanol to olefin (MTO) process
is based
io 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 US-A-4,238,631; US-A-4,328,384, US-A-
4,423,274.
is These patents further disclose the deposition of coke onto the zeolites in
order to
increase selectivity to light olefins and minimize the formation of C5+
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, US-A-4,499,327 discloses
that many
20 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 A) but small enough to exclude
isobutane (kinetic diameter of 5.0 A). A particularly preferred SAPO is SAPO-
34.
US-A-4,752,651 discloses the use of nonzeolitic molecular sieves (N~MS)
2s 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 US-A-5,126,308. 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 pm and
3o no more than 10% of the particles have a particle size greater than 2.0 pm
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.
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In contrast to this art, applicants have found that molecular sieves having
the
empirical formula (ELXAIyPZ)02 (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, AI
and P
s 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
to 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
is 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:
(ELXAIyP~)02
2o 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 AI 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
2s 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:
30 (ELxAIyPZ)02
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
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of EL and has a value of at least 0.005, "y" is the mole fraction of AI 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.
s 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
to microporous framework structure of A102, PO2 and EL02 tetrahedral units.
Generally
the ELAPOs have the empirical formula
(ELXAIyPZ)O2
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
is of EL and has a value of at least 0.005, "y" is the mole fraction of AI 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.
2o The preparation of various ELAPOs are well known in the art and may be
found
in US-A-: 4,554,143 (FeAPO); 4,440,871 (SAPO); 4,853,197 (MAPO, MnAPO, ZnAPO,
CoAPO); 4,793,984 (CAPO), 4,752,651 and 4,310,440. Generally, the ELAPO
molecular sieves are synthesized by hydrothermal crystallization from a
reaction
mixture containing reactive sources of EL, aluminum, phosphorus and a
templating
2s 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 puaternary
ammonium compounds. An especially preferred templating agent is
3o tetraethylammonium hydroxide (TEAOH).
The reaction mixture is placed in a sealed pressure vessel, optionally lined
with
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an inert plastic material such as polytetrafluoroethylene and heated
preferably under
autogenous pressure at a temperature between 50°C and 250°C and
preferably
between 100°C and 200°C for a time sufficient to produce
crystals of the ELAPO
molecular sieve. Typically the time varies from 1 hour to 120 hours and
preferably from
s 24 hours to 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
to the crystals have a plate morphology. Especially good selectivity (C2
versus C3-) 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
is 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.
2o 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
2s 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 (C2%C3
) is about
30 1.2, whereas when the smallest dimension is greater than 0.1 microns, the
ratio of
C2%C3 is 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
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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° to about 700°C until the template is removed, usually
a few hours.
A preferred embodiment of the invention is one in which the metal (EL) content
s varies from 0.005 to 0.05 mole fraction. If EL is more than one metal then
the total
concentration of all the metals is between 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.
Patent 4,440,871. Of the specific crystallographic structures described in the
'871
to 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 A.
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
15 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
2o 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
2s 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
3o Space Velocity (WHSV) based on methanol can vary from about 1 h~ 1 to about
1000
hr' and preferably from about 1 hr' to about 100 hr'.
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
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temperature of 300°C to 600°C, preferably from 400°C to
550°C and most preferably
from 450°C to 525°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 to
1724
kPa and preferably from 34 kPa to 345 kPa.
s 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
io from 5 to 90 mole percent of the feedstock and preferably from 25 to 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
15 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
2o 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
25 EXAMPLE 1
A series of molecular sieves (SAPOs) were prepared by the following procedure.
In a container orthophosphoric acid (85%) was combined with water. To this
there was
added a silica sol and a 35 wt.% aqueous solution of tetraethylammonium
hydroxide
(TEAOH). Finally, alumina in the form of pseudo-boehmite along with water and
3o SAPO-34 seed material were added and blended in. The resulting mixtures had
compositions in molar oxide ratios as set forth in Table 1 below.
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TABLE 1
Reaction
Mixture
Compositions
For SAPOs
Reaction
Sample Time TEAOH Si02 AI203 P2O5 H20
I. D.
A 48 1.0 0.10 1,0 1.0 35
B 48 1.0 0.10 1.0 1.0 35
C 48 1.0 0.10 1.0 1.0 45
D 24 1.0 0.10 1.0 1.0 45
E 36 1.0 0.15 1.0 1.0 40
F 48 1.0 0.20 1.0 ~ 1.~ _
45
The mixture was now placed in a steel pressure reactor equipped with a turbine
stirrer.
The mixture was now stirred and heated to 100°C over a 6 hour period,
held at 100°C
for 6 hours, then heated to 175°C over a period of 3 hours and held
there for the
s reaction time of 24, 36 or 48 hours. Finally, the reaction mixture was
cooled to ambient
temperature and the solid product recovered by centrifugation and washed with
water.
All the products were analyzed and found to be SAPO-34 molecular sieves.
EXAMPLE 2
The catalysts prepared in Example 1 were evaluated for the conversion of
to methanol to light olefins in a fixed bed pilot plant. A 4 gram sample in
the form of 20-40
mesh agglomerates was used for the testing. Before testing, each sample was
calcined
in air in a muffle oven at 650°C for 2 hours and then pre-treated in
situ by heating to
400°C for 1 hour under nitrogen. The pretreated sample was now
contacted with a
feed consisting of methanol and H20 in a 1/0.44 molar ratio at 435°C, 5
psig and 2.5 hr
is ' MeOH WHSV. The composition of fihe effluent was measured by an on-line GC
after
30 minutes on stream to determine initial conversion and selectivities.
Complete
conversion was obtained initially with all catalysts but it fell with time on
stream as the
catalysts deactivated. Table 2 presents the selectivity to ethylene and
propylene and
the ethylene/propylene product ratio at the point where conversion was 99% for
each
2o catalyst.
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TABLE 2
Effect of Crystal Dimension on Ethylene/Propylene Production
Average Smallest C2 +C3
Catalyst Dimension (Microns)Crystal MorphologySelectivity C2%C3
1.D. (%)
A 0.07 Thin lates 82.4 1.17
B 0.08 Plates 79.2 1.18
C 0.09 Thin Plates 82.2 1.25
D 0.13 Plates 80.8 1.40
E 0.17 Plates 81.2 1.41
F 0.58 Cubic 78.7 1.48
The average smallest crystallite dimension was determined by measuring 20
representative crystallites in one or more micrographs obtained using a
Scanning
s Electron Microscope at 30,OOOx magnification. The data indicate that when
the
smallest crystal dimension is greater than 0.1 micron and the crystal
morphology is
plates, a greater amount of ethylene is produced. It is also observed that
when the
crystal morphology is cubic and the smallest dimension is greater than 0.2
microns,
one obtains the highest production of ethylene. Note that when the smallest
to dimension is less than 0.1, one obtains poor results (greater propylene
production)
even though the crystal morphology is plates.
s
