Molecular sieve abstract
A method for converting starting material to olefins comprising
contacting the starting material with a small pore molecular sieve
catalyst 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 the group comprising Groups
VIB, VIIB, or VII or mixtures thereof.
Molecular sieve claims
What is claimed is:
1. A method for converting oxygenates to olefins comprising
contacting said oxygenates with a small pore molecular sieve catalyst
under production conditions effective to produce olefins;
wherein said molecular sieve catalyst comprises a framework, and
said framework is treated after synthesis with a transition metal
compound comprising transition metal ions under catalyst treatment
conditions effective to incorporate at least a portion of said transition
metal ions onto said framework, wherein said transition metal ions
are selected from the group consisting of Groups VIB, VIIB, or VIII
and mixtures thereof.
2. The method of claim 1 wherein said molecular sieve catalyst
is selected from the group consisting of a silicoaluminophosphate
(SAPO), ZSM-34 chabazite, erionite, and mixtures thereof.
3. The method of claim 2 wherein said silicoaluminophosphate is
selected from the group consisting of SAPO-17 SAPO-18 SAPO-34
and SAPO-44.
4. The method of claim 1 wherein said molecular sieve catalyst
comprises pores having a size greater than about 3.5 Angstroms and
less than about 5.0 Angstroms.
5. A method for converting oxygenates to olefins comprising
contacting said oxygenates with a small pore molecular sieve catalyst
under production conditions effective to produce olefins,
wherein said molecular sieve catalyst comprises a framework, and
said framework is treated after synthesis with a transition metal
compound comprising transition metal ions under catalyst treatment
conditions effective to incorporate at least a portion of said transition
metal ions onto said framework, wherein said transition metal ions
are selected from the group consisting of nickel, cobalt, and mixtures
thereof.
6. The method of claim 1 wherein said transition metal compound
is selected from the group consisting of halides, sulfates, acetates,
carbonyls, nitrates, and mixtures thereof.
7. The method of claim 1 wherein said method produces a modified
molecular sieve comprising a silicon to metal atomic ratio in the
range of from about 0.1:1 to about 1000:1.
8. The method of claim 1 wherein said production conditions comprise
a temperature of from about 200.degree. C. to about 600.degree.
C.
9. The process of claim 1 wherein said oygenates feed is 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, di-methyl carbonate, carbonyl
compounds, and mixtures thereof.
10. The method of claim 9 wherein said oygenates comprise methanol
or dimethyl ether.
11. The process of claim 1 wherein said oygenates further comprise
a diluent.
12. The method of claim 11 wherein the diluent is selected from
said group consisting of water, nitrogen, hydrogen, paraffins, olefins,
aromatics, and mixtures thereof.
13. The method of claim 12 wherein said diluent is selected from
the group consisting of water and nitrogen.
14. The method of claim 1 wherein said oxygenates are selected
from the group consisting of a halide, a mercaptan, a sulfide, and
an amine.
15. The method of claim 1 wherein said production conditions comprise
a pressure of from about 0.1 kPa to about 100 MPa.
16. The method of claim 1 wherein said production conditions comprise
a weight hourly space velocity in the range of from about 0.01 to
about 500 hr.sup.-1.
17. A method for converting oxygenates to olefins comprising:
contacting said oxygenates with a molecular sieve catalyst selected
from the group consisting of a SAPO, ZSM-34 a chabazite, an erionite,
and mixtures thereof, under production conditions effective to produce
olefins;
wherein said molecular sieve catalyst comprises a framework comprising
pores having a size less than about 5.0 Angstroms; and
wherein said framework is treated after synthesis with a transition
metal compound comprising transition metal ions selected from the
group consisting of nickel, cobalt, and mixtures thereof, under
catalyst treatment conditions effective to incorporate at least
a portion of said transition metal ions onto said framework.
Molecular sieve description
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to a process for the conversion of oxygenates
to hydrocarbons using small pore molecular sieve catalysts. More
particularly, this invention relates to a process for conversion
of oxygenates to olefins using silicoaluminophosphate molecular
sieve catalysts which have been incorporated with certain transition
metals after the synthesis of the molecular sieve.
2. Background Art of the Invention
Olefins have traditionally been produced through the process of
petroleum cracking. Because of the potential limited availability
and high cost of petroleum sources, the cost of producing olefins
from such petroleum sources has been steadily increasing. Light
olefins such as ethylene serve as feeds for the production of numerous
chemicals.
The search for alternative materials for the production of light
olefins, such as ethylene, has led to the use of oxygenates such
as alcohols, and more particularly to methanol and ethanol or their
derivatives as feedstocks. These and other alcohols may be produced
by fermentation or from synthesis gas. Synthesis gas can be produced
from natural gas, petroleum liquids, carbonaceous materials including
coal, recycled plastics, municipal wastes, or any organic material.
Thus, alcohol and alcohol derivatives may provide non-petroleum
based routes for hydrocarbon production.
It is well known in the prior art to convert oxygenates to olefins
by contacting the oxygenate with various types of catalysts. Medium
and large pore molecular sieve catalysts, such as borosilicate,
ZSM-5 SAPO-11 and SAPO-5 may be used. For example, U.S. Pat.
No. 4292458 teaches a process in which a crystalline borosilicate,
is converted to the hydrogen form, ion-exchanged with Ni(NO.sub.3).sub.2
in water, washed, and then calcined to give a catalyst useful for
conversion of methanol to ethylene and propylene (among other products).
Similarly, U.S. Pat. No. 4311865 teaches the use of the medium
pore zeolite, ZSM-5 (approximately 5.5 Angstroms) pore size of,
which is ion-exchanged with cobalt, and then calcined to produce
a catalyst which is then used to convert methanol to hydrocarbons
(including olefins). Both of these processes use ion-exchange to
add the metal to the medium pore molecular sieve. The following
references also teach the process of using the large pore catalyst
SAPO-5 (pore size of approximately 8.0 Angstroms), for conversion
of methanol to olefin; however, in these instances, the nickel is
incorporated during synthesis, rather than by the use of ion-exchange:
N. Azuma, et al., Nickel(I) Location and Adsorbate Interactions
in Nickel(II)-Exchanged Silicoaluminophosphate Type 5 As Determined
by Electronic Spin Resonance and Electron Spin Echo Modual Spectroscopies,
Journal of Physical Chemistry, Vol. 99 No. 17 pages 6670-6 (1995)
and V. Mavrodinova et al., Effect of the Introduction of Ni(II)--On
the Catalytic Properties of SAPO-5 Molecular Sieves, Zeolite Chemistry
and Catalysis, Pages 295-302 Elsevier Science Publishers B. V.
Amsterdam (1991).
Small pore catalysts such as SAPO-34 have been used to convert
methanol to olefins, as described in an article by T. lnui, Structure-Reactivity
Relationships in Methanol to Olefins Conversion in Various Microporous
Crystalline Catalysts, Structure-Activity and Selectivity Relationships
in Heterogensis Catalysts, pages 233-42 Elsevier Science Publishers
B. V. Amsterdam (1991). However, the conversion stability is not
as good as when using medium pore molecular sieves which have been
ion-exchanged with the metals. Based on the favorable effect of
metal addition to medium pore molecular sieve, it would seem that
this same effect would be seen using small pore molecular sieves.
However, until now, as taught by Inui, the metal ion had to be incorporated
into the catalyst during synthesis, rather than by post-synthesis
ion exchange.
Inui has confirmed that nickel substitution into SAPO-34 during
the synthesis process results in improving the selectivity of methanol
to ethylene. Inui's experiments tested three different SAPO-34 catalysts
for use in the conversion process. In the first, the catalyst without
any nickel substitution was used as a comparative sample. In the
second and the third, nickel was substituted during the synthesis
for a resulting silicon to nickel ratio of 100 and 40 respectively.
In all three experiments, a feed of 20% methanol and 80% nitrogen
diluent was used. The reactions were carried out at a total pressure
of one atmosphere (0.1 MPa), a temperature of 450.degree. C., and
a gas hourly space velocity (GHSV) of 2000 hr.sup.-1.
The experiments teach that the ethylene yield will increase from
30% to 60% by using the Ni-SAPO-34 catalyst with the Si/Ni ratio
of 100 as compared to the untreated SAPO-34 catalyst. Inui's use
of the Ni-SAPO-34 catalyst with the Si/Ni ratio of 40 increased
the ethylene yield from 30% to 90% as compared to the untreated
SAPO-34 catalyst. The combined ethylene and proplyene yield was
also increased on an absolute basis by 25% and 34%, respectively.
One can see that the nickel incorporation has a definite impact
upon the ethylene and propylenes yields. However, even though Inui
has demonstrated that nickel substitution is attractive, the nickel
was substituted during the catalyst synthesis process. Metal substitution
via catalyst synthesis generally requires elevated temperatures,
elevated pressures, and special equipment, therefore making it commercially
less attractive. In contrast, post-synthesis metal incorporation
can be carried out under milder conditions. In addition, the physical
characteristics, such as particle size, can be varied prior to metal
incorporation to achieve greater flexibility, thus allowing a wider
range of operating parameters with which to achieve the incorporation.
Therefore, in view of the problems associated with incorporating
the metal during synthesis, it would be commercially useful and
desirable to be able to produce such a catalyst by incorporating
after molecular sieve synthesis, rather than during synthesis. A
post-synthesis technique would provide flexibility in catalyst preparation,
choice of metal additive, metal concentration, and selection of
molecular sieves.
SUMMARY OF INVENTION
This invention provides a method for converting oxygenates to olefins,
comprising contacting the starting material with a small pore molecular
sieve catalyst under effective conversion conditions to provide
olefins wherein the molecular sieve has been modified after synthesis
by incorporation of a transition metal (from Groups VIB, VIIB, or
VII as defined by the CAS Version in the CRC Handbook of Chemistry
and Physics, 74th edition, 1993) originating from a metal compound.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is characterized by use of a small pore molecular
sieve catalyst, which has a transition metal incorporated after
molecular sieve synthesis, rather than during synthesis, in a process
for convating oxygenates to olefins.
For this application, the molecular sieve may be a zeolite, such
as ZSM-34 a silicoaluminophosphate (SAPO), a chabazite, an erionite,
and mixtures thereof, preferably, but not limited to, a SAPO catalyst.
In the present invention, small pore molecular sieves are defined
as having a pore size of less than about 5.0 Angstrom units. Generally,
suitable catalysts have a pore size ranging from about 3.5 to about
5.0 Angstroms, preferably from about 4.0 to about 5.0 Angstroms,
and most preferably from about 4.3 to about 5.0 Angstroms.
Zeolite materials, both natural and synthetic, have been demonstrated
to have catalytic properties for various types of hydrocarbon conversion
processes. In addition, zeolite materials have been used as adsorbents,
catalyst carriers for various types of hydrocarbon conversion processes,
and other applications. Zeolites are complex crystalline aluminosilicates
which form a network of AlO.sup.-.sub.2 and SiO.sub.2 tetrahedra
linked by shared oxygen atoms. The negativity of the tetrahedra
is balanced by the inclusion of cations such as alkali or alkaline
earth metal ions. In the manufacture of some zeolites, non-metallic
cations, such as tetramethylammonium (TMA) or tetrapropylammonium
(TPA), are present during synthesis. The interstitial spaces or
channels formed by the crystalline network enable zeolites to be
used as molecular sieves in separation processes, as catalyst for
chemical reactions, and as catalyst carriers in a wide variety of
hydrocarbon conversion processes.
Zeolites include materials containing silica and optionally alumina,
and materials in which the silica and alumina portions have been
replaced in whole or in part with other oxides. For example, germanium
oxide, tin oxide, and mixtures thereof can replace the silica portion.
Boron oxide, iron oxide, gallium oxide, indium oxide, and mixtures
thereof can replace the alumina portion. Unless otherwise specified,
the terms "zeolite" and "zeolite material" as
used herein, shall mean not only materials containing silicon atoms
and, optionally, aluminum atoms in the crystalline lattice structure
thereof, but also materials which contain suitable replacement atoms
for such silicon and aluminum atoms.
SAPO's have a three-dimensional microporous crystal framework structure
of PO.sub.2.sup.+, AlO.sub.2.sup.-, and SiO.sub.2 tetrahedral units.
The chemical composition (anhydrous) is:
wherein "R" represents at least one organic templating
agent present in the intracrystalline pore system: "m"
represents the moles of "R" present per mole of (Si.sub.x
Al.sub.y P.sub.z)O.sub.2 and has a value of from zero to 0.3 the
maximum value in each case depending upon the molecular dimensions
of the templating agent and the available void volume of the pore
system of the particular SAPO species involved, and "x",
"y", and "z" represent the mole fractions of
silicon, aluminum and phosphorus, respectively. Typical small pore
SAPO's are SAPO-17 SAPO-18 SAPO-34 SAPO-44 and others. "R"
may be removed at elevated temperatures.
The metal which may be employed in the incorporation process is
a transition metal selected from Groups VIB, VIIB, or VIII or mixtures
thereof, as defined by the CAS Version of the CRC Handbook of Chemistry
and Physics, 74th edition, 1993. Preferably, the metal is either
nickel or cobalt.
The metal containing compounds which may be used in the present
invention may be of various compositions, i.e. in the form of the
corresponding halide, sulfate, acetate, carbonyl, nitrate, or mixtures
thereof. When the desired catalyst comprises SAPO-34 and the metal
is nickel, it is preferable to use the hydrated form of nickel acetate
as the metal containing compound.
The process of incorporating the transition metal may be accomplished
through any one of the standard methods well known to those skilled
in the art. In one embodiment, a solution of the desired metal is
first made by dissolving the desired amount of the metal containing
compound in water under mild conditions. Preferably the water is
de-ionized. The temperature of mixing is dependent upon the solubility
of the metal compound in water, or whatever other medium is selected.
The process may be conducted under pressure or at atmospheric pressure.
After adequate mixing, the solution is then added to the selected
amount of the molecular sieve. The resulting mixture is stirred
as required. In some cases, stirring is not required and the mixture
may be left undisturbed for a time adequate to permit the desired
level of metal incorporation. The catalyst product is then filtered,
optionally washed, dried, and calcined by methods well known to
those skilled in the art.
The amount of metal which is incorporated onto the molecular sieve
may vary over a wide range depending, at least in part, on the selected
molecular sieve catalyst and on the incorporation method. The amount
of metal incorporated is measured on an atomic metal basis in terms
of silicon to metal ratio. The silicon to metal atomic ratios are
in the range from about 0.01:1 to about 1000:1 preferably from
about 0.1:1 to about 500:1 and most preferably from about 5:1 to
about 50:1.
The conversion process employs a starting material (feedstock)
comprising "oxygenates". As used herein, the term "oxygenates"
is intended to comprise aliphatic alcohols, ethers, carbonyl compounds
(aldehydes, ketones, carboxylic acids, carbonates, and the like)
along with those compounds containing hetero-atoms, e.g., halides,
mercaptans, sulfides, amines, and mixtures thereof. The aliphatic
moiety preferably contains from about 1 to about 10 carbon atoms
and more preferably contains from about 1 to about 10 carbon atoms.
Representative oxygenates include, but are not limited to, lower
straight or branched chain aliphatic alcohols, their unsaturated
counterparts and the nitrogen, halogen and sulfur analogues of such.
Examples of suitable compounds include, but are not limited to:
methanol; ethanol; n-propanol; isopropanol; C.sub.4 -C.sub.20 alcohols;
methyl ethyl ether; dimethyl ether; diethyl ether; di-isopropyl
ether; methyl mercaptan; methyl sulfide; methyl amine; ethyl mercaptan;
di-ethyl sulfide; di-ethyl amine; ethyl chloride; formaldehyde;
di-methyl carbonate; di-methyl ketone; acetic acid; n-alkyl amines,
and n-alkyl halides, n-alkyl sulfides, each having n-alkyl groups
of 3 to 10 carbon atoms; and mixtures thereof. The term "oxygenate"
as employed herein designates only the organic material used as
the feed. The total charge of feed to the reaction zone may contain
additional compounds such as diluents.
The conversion process is preferably carried out in the vapor phase
such that the feedstock is contacted with the defined molecular
sieve catalyst at effective process conditions in a vapor phase
in a reaction zone so as to produce the desired olefins. Effective
process conditions include, but are not necessarily limited to an
effective temperature, pressure, WHSV (Weight Hourly Space Velocity)
and, optionally, an effective amount of diluent, correlated to produce
olefins. Alternatively, the process may be carried out in a liquid
phase. When the process is carried out in the liquid phase, different
conversions and selectivities of feedstock-to-product may result
with respect to the relative ratios of the light olefin products
as compared to those formed by the vapor phase process.
The temperature which may be employed in the conversion process
may vary over a wide range depending, at least in part, on the selected
molecular sieve catalyst. The process is conducted at an effective
temperature range from about 200.degree. C. to about 700.degree.
C., preferably from about 250.degree. C. to about 600.degree. C.,
and most preferably from about 300.degree. C. to about 500.degree.
C. Temperatures outside the stated preferred ranges are not excluded,
although they do not fall within certain desirable embodiments of
the present invention. At the lower end of the temperature range,
and thus, generally, at a lower rate of reaction, the formation
of the desired light olefin products may become markedly slow. At
the upper end of the temperature range and beyond, the process may
not form an optimum amount of light olefin products.
The process is effectively carried out over a wide range of pressures
including autogeneous pressures. At pressures in the range from
about 0.1 kPa to about 100 MPa, the formation of light olefin products
will be effected although the optimum amount of product will not
necessarily form at all pressures. The preferred pressure is in
the range from about 6.9 kPa to about 34 Mpa, with the most preferred
range being from about 48 kPa to about 0.34 MPa. The pressures referred
to herein for the process are exclusive of the inert diluent, if
any, that is present, and refer to the partial pressure of the feedstock
as it relates to oxygenate compounds and/or mixtures thereof. Pressures
outside the stated range are not excluded from the scope of this
invention, although such do not fall within certain desirable embodiments
of the invention. At the lower and upper end of the pressure range,
and beyond, the selectivities, conversions and /or rates of convasion
to light olefin products may not occur at the optimum, although
light olefins such as ethylene may still be formed.
The process is effected for a period of time sufficient to produce
the desired olefin products. In general, the residence time employed
to produce the desired product can vary from seconds to a number
of hours. It will be readily appreciated that the residence time
will be determined to a significant extent by the reaction temperature,
the pressure, the molecular sieve selected, the WHSV, the phase
(liquid or vapor), and the process design characteristics selected.
The process is effectively carried out over a wide range of WHSV
for the feedstock and is generally in the range from about 0.01
hr.sup.-1 to about 500 hr.sup.-1 preferably from about 0.1 hr.sup.-1
to about 200 hr.sup.-1 and most preferably from about 0.5 hr.sup.-1
and 100 hr.sup.-1. As the catalyst may contain other materials which
act as inerts, the WHSV is calculated on the weight basis of methanol
and small pore molecular sieve used.
The conversion process may optionally be carried out in the presence
of one or more inert diluents which may be present in the feedstock
for example in an amount between 1 and 99 molar percent, based on
the total number of moles of all feed and diluent components fed
to the reaction zone (or catalyst). Typical of diluents which may
be employed in the instant process are helium, argon, nitrogen,
carbon monoxide, carbon dioxide, hydrogen, water, paraffins, hydrocarbons
(such as methane), aromatic compounds, and mixtures thereof. The
preferred diluents are water and nitrogen.
The olefin production process may be carried out in a batch, semi-continuous
or continuous fashion. The process can be conducted in a single
reaction zone or a number of reaction zones arranged in series or
in parallel, or may be conducted intermittently or continuously
in an elongated tubular zone or in a number of such zones. When
multiple reaction zones are employed, it may be advantageous to
employ one or more of the defined small pore molecular sieves in
series to provide for a desired product mixture.
Owing to the nature of the process, it may be desirous to carry
out the process of the present invention using the molecular sieve
catalysts in a dynamic bed system or any system of a variety of
transport beds rather than in a fixed bed system. Such systems would
readily provide for any regeneration (if required) of the molecular
sieve catalyst after a given period of time. If regeneration is
required, the molecular sieve catalyst can be continuously introduced
as a moving bed to a regeneration zone where it can be regenerated,
such as for example by removing carbonaceous materials or by oxidation
in an oxygen-containing atmosphere. In the preferred practice of
the invention, the catalyst will be subject to a regeneration step
by burning off carbonaceous deposits accumulated during the conversion
reactions.
The following examples illustrate, but do not limit, the present
invention.
EXAMPLES
Catalysts were prepared and then tested for methanol conversion.
SAPO-34 was prepared according to U.S. Pat. No. 4440871 to provide
a basis for comparision with the examples of transition metal incorporated
small pore molecular sieve catalsyts as prepared by the methods
as described above.
Example I
Ni-SAPO-34 was prepared as follows. A nickel containing solution
was prepared by dissolving 0.26 g of Ni(OAc).sub.2 .cndot.4H.sub.2
O in 23 cc of de-ionized water at room temperature. This solution
was added to 3.92 grams of SAPO-34 and the mixture was stirred at
room temperature for two hours. The finished catalyst was filtered,
and then dried at 110.degree. C. for 4 hours.
The resulting dried catalyst was then calcined at 550.degree. C.
for 16 hours. The silicon to nickel atomic ratio measured in terms
of an atomic weight basis was about 1:10. |