Molecular sieve abstract
A method for converting starting material to olefins comprising
contacting the starting material with a small pore non-zeolitic
molecular sieve catalyst under effective conditions to produce olefins,
wherein the non-zeolitic molecular sieve has been prepared in-situ
or modified after synthesis by incorporation using an alkaline earth
metal compound, wherein the alkaline earth metal ion is selected
from the group consisting of strontium, calcium, barium, and mixtures
thereof.
Molecular sieve claims
What is claimed is:
1. A method for converting an oxygenate to an olefin comprising
contacting said oxygenates with a non-zeolitic molecular sieve catalyst
under effective conditions to produce said olefin, wherein said
non-zeolitic molecular sieve has a pore diameter size of about 5
Angstroms or less and has been prepared in-situ or modified after
molecular sieve synthesis by incorporation of an alkaline earth
metal selected from the group consisting of strontium, calcium,
barium, and mixtures thereof, using an alkaline earth metal ion
containing compound, wherein the alkaline earth metal ion in said
alkaline earth metal ion containing compound is selected from the
group consisting of strontium, calcium, barium, and mixtures thereof.
2. The method of claim 1 wherein said molecular sieve catalyst
is selected from the group consisting of a silicoaluminophosphate
(SAPO), aluminophosphate (ALPO), 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
SAPO-44 and SAPO-56.
4. The method of claim 1 wherein said molecular sieve catalyst
pore size is greater than 3.5 Angstroms and less than about 5.0
Angstroms.
5. The method of claim 1 wherein said alkaline earth metal is strontium
or calcium.
6. A method of claim 1 wherein said alkaline earth metal compound
is selected from the group consisting of halides, sulfates, formates,
acetates, alkoxides, carbonyls, nitrates, or mixtures thereof.
7. A method of claim 1 wherein said modified non-zeolitic molecular
sieve has a metal to silicon atomic ratio in the range of from 0.01:1
to about 2:1.
8. The method of claim 1 wherein the method for converting said
oxygenate to said olefin is conducted at a temperature of from 200.degree.
C. to about 600.degree. C.
9. The method of claim 1 wherein said oxygenate 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 oxygenate comprises methanol
or dimethyl ether.
11. The method of claim 1 wherein said oxygenate includes a diluent.
12. The method of claim 11 wherein the diluent is selected from
the group consisting of water, nitrogen, hydrogen, paraffins, aromatics,
and mixtures thereof.
13. The method of claim 12 wherein the diluent is water or nitrogen.
14. The method of claim 1 wherein said oxygenate comprises a halide,
a mercaptan, a sulfide, or an amine.
15. The method of claim 1 wherein said method for converting said
oxygenate to said olefin is conducted at a pressure of from 0.1
kPa to about 100 MPa.
16. The method of claim 1 wherein said method for converting said
oxygenate to said olefin using said treated molecular sieve is conducted
at a WHSV in the range of from 0.01 to about 500 hr.sup.-1.
17. The method of claim 1 wherein said non-zeolitic molecular
sieve comprises SAPO-34 and strontium acetate.
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 non-zeolitic molecular sieve catalysts.
More particularly, this invention relates to a catalyst composition,
a method to prepare such a catalyst, and a process to use such a
catalyst in a process for conversion of oxygenates to olefins using
silicoaluminophosphate molecular sieve catalysts which have been
incorporated with certain alkaline earth metals either during or
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. Large,
medium, and small pore, zeolitic and non-zeolitic, molecular sieve
catalysts may be used.
It is also well known in the prior art that molecular sieves of
various pore diameters and compositions have been treated by addition
of alkaline earth metals to improve catalyst performance for use
in various applications. It is also well known that when comparing
the performance of two catalysts, even if every physical parameter
of each of the catalysts is the same, that if the two catalysts
have a different composition, then one cannot predict based on the
performance of one catalyst, how the second catalyst will perform,
U.S. Pat. No. 4752651 col. 2 lines 31-68. So even if a particular
alkaline earth metal has been added to one type of catalyst for
a particular use, it does not mean that the same metal will have
the same beneficial effect on the performance of the second catalyst.
Even though the art teaches the use of some of the alkaline earth
metals to improve the performance of large, medium and small pore
zeolites, it fails to teach the use of all such alkaline earth metals,
including strontium, calcium, and barium, to improve the performance
of non-zeolitic molecular sieve catalysts with diameters of less
than about 5 Angstroms for the use in oxygenate conversion.
U.S. Pat. No. 4752651 teaches the modification of small pore
non-zeolitic molecular sieve catalysts using the alkaline earth
metals of beryllium and magnesium. However, the prior art fails
to teach and/or enable either the incorporation of the alkaline
earth metals of strontium, calcium, and barium into small pore molecular
sieves or the in situ process of such metals into such a catalyst
for the use in oxygenate conversion.
This failure to teach may be due to larger ionic radii of the cations
with the higher atomic numbers in Group IIA. For example, beryllium
and magnesium each have a size of 0.31 and 0.65 Angstroms, respectively.
This is to be contrasted with the larger sizes of calcium, strontium,
and barium with ionic radii of 0.99 1.13 and 1.35 Angstroms, respectively.
Based on this size difference, one of ordinary skill in the art
would not think that these larger radii ions could be used as effectively
in modifying a small pore catalyst. Even though all of these radii
are less than 5 Angstroms, it is well known that the ions exist
in the solvated form with the solvent molecules attached. Therefore,
even though the metal ion has a radius of less than 5 Angstroms,
that in the solvated form, the effective radius will be much larger.
Meanwhile, JP94074134 (JP01051316) discloses an in situ process
which appears to be a method to make a small pore aluminophosphosilicate
containing any one of the alkaline earth metals which is useful
in an oxygenate conversion process. However, upon a close reading
of the disclosure, this patent actually teaches the use of a medium
pore catalyst, such as ZSM5 and not a small pore catalyst, such
as SAPO-34 for oxygenate conversion. The disclosure focuses on
how their catalyst is unique compared to a conventional ZSM-5. For
example, their catalyst has a pore diameter of 5 to 6 Angstroms
and an adsorption volume that is similar to that of common ZSM-5
type zeolites. The x-ray pattern of their material is similar to
medium pore sized ZSM-5 and not similar to that of small pore sized
SAPO-34. Their catalyst is described as a novel zeolite that has
a pore diameter that is between large diameter zeolites, such as
faujasite X and Y types, and small diameter zeolites, such as erionite
and offretite which further distinguishes their catalyst from a
small pore molecular sieve.
Therefore, based on the teachings of the prior art, it is surprising
to learn that the alkaline earth metals of strontium, calcium, or
barium can be successfully added to a small pore non-zeolitic molecular
sieve for enhancement of the performance for such a catalyst for
use in the oxygenate conversion process.
SUMMARY OF INVENTION
This invention provides a catalyst, a method to prepare the catalyst,
and a method for converting a starting material to olefins, comprising
contacting the starting material with a small pore non-zeolitic
molecular sieve catalyst under effective conversion conditions to
provide olefins wherein the molecular sieve has been prepared in-situ
or modified after synthesis by incorporation of one or more of the
alkaline earth metals of strontium, calcium, or barium, originating
from a corresponding metal compound.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is the X-ray diffraction pattern for the strontium silicoaluminophosphate
(Sr-SAPO) sample which was prepared per Example III as defined below.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is characterized by use of a small pore non-zeolitic
molecular sieve catalyst, which has one or more of the alkaline
earth metals, selected from the group consisting of strontium, calcium,
barium, incorporated onto the molecular sieve either during or after
synthesis, in a process for the conversion of starting material
to olefins.
For this application, the non-zeolitic molecular sieve may be a
silicoaluminophosphate (SAPO), an aluminophosphate (ALPO), and mixtures
thereof, preferably, but not limited to, a SAPO catalyst. In the
present invention, small pore non-zeolitic 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 units, preferably from about 4.0 to about
5.0 Angstroms, and most preferably from about 4.3 to about 5.0 Angstroms.
Non-zeolitic materials have been demonstrated to have catalytic
properties for various types of hydrocarbon conversion processes.
In addition, non-zeolitic materials have been used as adsorbents,
catalyst carriers for various types of hydrocarbon conversion processes,
and other applications. Non-zeolitic molecular sieves are complex
three dimensional crystalline structures which include either AlO.sub.2.sup.-
or SiO.sub.2 or both AlO.sub.2.sup.- and SiO.sub.2 and a third metal
oxide. The interstitial spaces or channels formed by the crystalline
network enable non-zeolites to be used as molecular sieves in separation
processes and catalysts for chemical reactions and catalyst carriers
in a wide variety of hydrocarbon conversion processes.
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 SAPO-56 and others.
"R" may be removed at elevated temperatures.
ALPO's have a three-dimensional microporous crystal framework structure
of PO.sub.2.sup.+ and AlO.sub.2.sup.- 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 (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 "y" and "z"
represent the mole fractions of aluminum and phosphorus, respectively.
"R" may be removed at elevated temperatures.
The metal which may be employed in either the in situ or incorporation
process is an alkaline earth metal selected from group consisting
of strontium, calcium, barium, and mixtures thereof. Preferably,
the metal is either strontium or calcium, and most preferably, the
metal is strontium.
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, formate, acetate, alkoxide, carbonyl,
nitrate, or mixtures thereof. When the desired catalyst comprises
SAPO-34 and the metal is strontium, it is preferable to use the
hydrated form of strontium acetate as the metal containing compound.
The process of making the catalyst in-situ may be accomplished
through any one of the standard methods well known to those skilled
in the art including, but not limited to, hydrothermal synthesis
under autogenic pressure at elevated temperatures. Typical precursors
include, but are not limited to, aluminum oxide, aluminum trimethoxide,
and aluminum triethoxide as the source of aluminum. Orthophosphoric
acid, trimethyl phosphate, and triethyl phosphate are examples of
typically used precursors for phosphorus. Colloidal silica, silica
sol, silicon tetramethoxide, and silicon tetraethoxide are examples
of typically used precursors for silica. Templates which are often
used in the synthesis process, include, but are to limited to, tetramethylammonium
hydroxide and tetraethylammonium hydroxide.
In one embodiment, a reaction mixture is first prepared by mixing
the desired amounts of the selected aluminum oxide and selected
phosphoric acid with vigorous stirring. Next, de-ionized water and
the desired amount of silica sol is added and the entire mixture
is continued to be stirred to achieve complete mixing. Then, the
selected organic template is added to this mixture and the resultant
catalyst mixture is completely mixed by additional stirring. An
aqueous solution containing the desired metal is then added to the
mixture.
The aqueous solution of the desired metal is 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. Alternatively, a medium other than water may be selected.
The process may be conducted under pressure or at atmospheric pressure.
The resultant catalyst 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 incorporation.
The catalyst product is finally filtered, optionally washed, dried,
and calcined by methods well known to those skilled in the art.
The process of taking the molecular sieve and incorporating the
selected alkaline earth metal post synthesis may be accomplished
through any one of the standard methods well known to those skilled
in the art including, but not limited to, incipient wetness methods,
ion-exchange, and mechanical mixing. 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 incorporation. The catalyst product is then filtered, optionally
washed, dried, and calcined by methods well known to those skilled
in the art.
For either method of preparation, either in-situ or post-synthesis,
the amount of metal which is incorporated into the molecular sieve
may vary over a wide range depending, at least in part, on the method
of preparation, the selected molecular sieve catalyst, and the incorporation
method.
The resulting composition of the prepared Sr-, Ca-, and Ba-SAPO's
may be expressed as follows:
wherein, the ratio of a to b=0.01 to 2
b=0.01 to 0.3
c=at least 0.05
d=at least 0.05
b+c+d=1.0
the ratio of b to c is less than 0.8
x=to balance the charge,
m=0 to 100 and
M=the alkaline earth metal of Sr, Ca, or Ba.
The amount of metal incorporated is measured on an atomic metal
basis in terms of metal to silicon ratio. The metal to silicon atomic
ratios are in the range from about 0.01:1 to about 2:1 preferably
from about 0.05:1 to about 1.5: 1 and most preferably from about
0.1:1 to about 1:1.
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 also those compounds containing hetero-atoms, e.g., halides,
mercaptans, sulfides, amines, and mixtures thereof. The aliphatic
moiety preferably contains from 1 to 10 carbon atoms and more preferably
contains 1 to 4 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; and n-alkyl amines, 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 in a vapor phase in a reaction
zone with the defined molecular sieve catalyst at effective process
conditions so as to produce the desired olefins, i.e., 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 that 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 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 ma y be carried out in a batch, semi-continuous
or continuous fashion. Th e process can be conducted in a single
reaction zone or a number of reaction zones arranged in series or
in parallel, or it may be conducted intermittently or continuously
in an elongated tubular zone or 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 by use of 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. |