Molecular sieve abstract
A method for modifying a molecular sieve catalyst to reduce methane
production during conversion of oxygenates to olefins. The molecular
sieve catalyst comprises a separately synthesized framework comprising
a material selected from the group consisting of silica, alumina,
phosphate, and combinations thereof. The framework is contacted
with a modifier, in the absence of a salt comprising a metal selected
from the group consisting of manganese, zirconium, and hafnium,
under conditions effective to modify the framework but insufficient
to dealuminize or deironize the framework. The modifier comprises
an oxygenated chelating agent comprising a parent compound which
is readily removable from the framework by calcination to leave
the modified molecular sieve catalyst. Also encompassed are the
modified molecular sieve catalysts produced by the method, and a
method of using the modified molecular sieve catalysts to reduce
methane production during conversion of oxygenates to olefins.
Molecular sieve claims
We claim:
1. A method for reducing methane production during conversion of
an oxygenate to olefins, said method comprising:
contacting a feed comprising said oxygenate with a modified molecular
sieve catalyst under conditions effective to produce olefins;
wherein said modified molecular sieve catalyst comprises a separately
synthesized microporous framework comprising a material selected
from the group consisting of silicon, aluminum, phosphate, and combinations
thereof; and,
wherein said separately synthesized microporous framework undergoes
modification by contact with a modifier in the absence of a salt
comprising a metal selected from the group consisting of manganese,
zirconium, and hafnium, under conditions effective to produce a
modified framework but insufficient to perform either function selected
from the group consisting of dealuminizing and deironizing said
framework;
wherein said modified framework comprises an amount of an oxygenated
chelating agent comprising a parent compound which is readily removed
from said framework by calcination; and
wherein, absent said modification said framework produces a first
quantity of methane during said contacting, and after said modification
said modified framework produces a second quantity of methane during
said contacting, said second quantity of methane being less than
said first quantity of methane.
2. The method of claim 1 wherein said oxygenated chelating agent
comprises functional groups selected from the group consisting of
an anhydride, a plurality of carboxyl groups, a combination of a
hydroxyl group and a carboxyl group, and salts of said functional
groups.
3. The method of claim 2 wherein said microporous framework comprises
pores consisting essentially of a diameter in the range of from
about 5 to about 10 Angstroms.
4. The method of claim 2 wherein said microporous framework comprises
pores consisting essentially of a diameter less than about 5 Angstroms.
5. The method of claim 4 wherein said molecular sieve catalyst
is a silicoaluminophosphate catalyst.
6. The method of claim 1 wherein said modifier is selected from
the group consisting of oxalic acid, maleic acid, maleic anhydride,
glutaric acid, glutar anhydride, adipic acid, EDTA, o-phthalic acid,
o-pht tartaric acid, salts thereof, and combinations thereof.
7. The method of claim 1 wherein said microporous framework comprises
pores consisting essentially of a diameter in the range of from
about 5 to about 10 Angstroms.
8. The method of claim 1 wherein said microporous framework comprises
pores consisting essentially of a diameter less than about 5 Angstroms.
9. The method of claim 8 wherein said molecular sieve catalyst
is a silicoaluminophosphate catalyst.
10. A method for reducing methane production during conversion
of an oxygenate to olefins, said method comprising:
contacting a feed comprising said oxygenate with a modified molecular
sieve catalyst under conditions effective to produce olefins;
wherein said modified molecular sieve catalyst comprises
a separately synthesized microporous framework comprising a material
selected from the group consisting of silicon, aluminum, phosphate,
and combinations thereof; and,
functional groups selected from the group consisting of oxygenated
chelating functional groups and derivatives thereof, incorporated
onto said separately synthesized microporous framework in an amount
sufficient to reduce methane production during conversion of said
oxygenate to olefins, wherein materials incorporated onto said synthesized
microporous framework comprise materials other than those selected
from the group consisting of manganese, zirconium, and hafnium.
11. The method of claim 10 wherein said oxygenated chelating agent
comprises functional groups selected from the group consisting of
an anhydride, a plurality of carboxyl groups, a combination of a
hydroxyl group and a carboxyl group, and salts of said functional
groups.
12. The method of claim 11 wherein said microporous framework comprises
pores consisting essentially of a diameter in the range of from
about 5 to about 10 Angstroms.
13. The method of claim 11 wherein said microporous framework comprises
pores consisting essentially of a diameter less than about 5 Angstroms.
14. The method of claim 13 wherein said molecular sieve catalyst
is a silicoaluminophosphate catalyst.
15. The method of claim 10 wherein said microporous framework comprises
pores consisting essentially of a diameter in the range of from
about 5 to about 10 Angstroms.
16. The method of claim 10 wherein said microporous framework comprises
pores consisting essentially of a diameter less than about 5 Angstroms.
17. The method of claim 16 wherein said molecular sieve catalyst
is a silicoaluminophosphate catalyst.
Molecular sieve description
FIELD OF THE INVENTION
The present invention is directed to a method for modifying molecular
sieve catalysts, to the molecular sieve catalysts produced by such
method, and to a method for using the modified molecular sieve catalysts
to reduce methane production during the conversion of oxygenates
to olefins.
BACKGROUND OF THE INVENTION
Light olefins (defined as "ethylene, propylene, and butylene")
serve as feeds for the production of numerous chemicals. Light olefins
traditionally are produced by petroleum cracking. Because of the
limited supply and/or the high cost of petroleum sources, the cost
of producing olefins from petroleum sources has increased steadily.
Alternative feedstocks for the production of light olefins are
oxygenates, such as alcohols, particularly methanol, dimethyl ether,
and ethanol. Alcohols may be produced by fermentation, or from synthesis
gas derived from natural gas, petroleum liquids, carbonaceous materials,
including coal, recycled plastics, municipal wastes, or any organic
material. Because of the wide variety of sources, alcohol, alcohol
derivatives, and other oxygenates have promise as an economical,
non-petroleum source for olefin production.
The catalysts used to promote the conversion of oxygenates to olefins
are molecular sieve catalysts. Because ethylene and propylene (herein
defined as "light olefins") are the most sought after
products of such a reaction, research has focused on what catalysts
are most selective to light olefins. Methods also are needed for
reducing the production of unwanted by-products, such as methane.
SUMMARY OF THE INVENTION
The present invention provides a method for modifying a molecular
sieve catalyst to reduce methane production during conversion of
oxygenates to olefins. The method comprises: providing a separately
synthesized framework comprising a material selected from the group
consisting of silica, alumina, phosphate, and combinations thereof,
contacting the separately synthesized microporous framework with
a modifier, in the absence of a salt comprising a metal selected
from the group consisting of manganese, zirconium, and hafnium,
under conditions effective to modify the framework but insufficient
to perform a function selected from the group consisting of dealuminizing
and deironizing the framework; wherein the modifier comprises an
oxygenated chelating agent comprising a parent compound which is
readily removable from the framework by calcination to leave the
modified molecular sieve catalyst.
DETAILED DESCRIPTION OF THE INVENTION
In the conversion of oxygenates to light olefins (defined herein
as ethylene and propylene), it is desirable to maximize the production
of light olefins and to minimize the production of undesired by-products,
such as methane, ethane, propane, carbon dioxide, hydrogen gas,
and C.sub.4 + materials, including aromatics. The present invention
minimizes the production of methane during such conversion by contacting
the molecular sieve catalyst used to promote the conversion with
a modifier comprising an oxygenated chelating agent comprising a
parent compound which is readily removable from the framework by
calcination.
Molecular sieve catalysts generally comprise a crystalline, three
dimensional, stable framework enclosing cavities of molecular dimensions.
The cavities form a well-defined microporous system of channels
and cages. The cavities or "pores" in a given type of
molecular sieve have well-defined dimensions which will only allow
molecules up to a certain size to enter the pores.
The present invention should reduce the production of methane by
substantially any molecular sieve catalyst, regardless of pore size.
Preferred molecular sieve catalysts for modification according to
the present invention are "small" and "medium"
pore molecular sieve catalysts. "Small pore" molecular
sieve catalysts are defined as catalysts with pores having a diameter
of less than about 5.0 Angstroms. "Medium pore" molecular
sieve catalysts are defined as catalysts with pores having a diameter
in the range of from about 5 to about 10 Angstroms.
One group of suitable molecular sieve catalysts is the zeolite
group. Several types of zeolites exist, each of which exhibit different
properties and different utilities. Structural types of zeolites
that are suitable for use in the present invention with varying
levels of effectiveness include, but are not necessarily limited
to AEI, AFT, APC, ATN, ATT, ATV, AWW, BIK, CAS, CHA, CHI, DAC, DDR,
EDI, ERI, GOO, KFI, LEV, LOV, LTA, MON, PAU, PHI, RHO, ROG, and
THO and substituted examples of these structural types, as described
in W. M. Meier and D. H. Olsen, Atlas of Zeolite Structural Types
(Butterworth Heineman-3rd ed. 1997), incorporated herein by reference.
Structural types of medium pore molecular sieves useful in the present
invention include, but are not necessarily limited to, MFI, MEL,
MTW, EUO, MTT, HEU, FER, AFO, AEL, TON, and substituted examples
of these structural types, as described in the Atlas of Zeolite
Types, previously incorporated herein by reference.
Preferred zeolite catalysts for use in the present invention include,
but are not necessarily limited to, ZSM-5 ZSM-34 erionite, and
chabazite.
Silicoaluminophosphates ("SAPO's") are another group
of molecular sieve catalysts that are useful in the invention. SAPO's
have a three-dimensional microporous crystal framework of PO.sub.2.sup.+,
AlO.sub.2 -, and SiO.sub.2 tetrahedral units. Suitable SAPO's for
use in the invention include, but are not necessarily limited to
SAPO-34 SAPO-17 and SAPO-18. A preferred SAPO is SAPO-34 which
may be synthesized according to U.S. Pat. No. 4440871 incorporated
herein by reference, and Zeolites, Vol. 17 pp. 512-522 (1996),
incorporated herein by reference.
SAPO's with added substituents also may be useful in the present
invention. These substituted SAPO's form a class of molecular sieves
known as "MeAPSO's." Substituents may include, but are
not necessarily limited to nickel, cobalt, strontium, barium, and
calcium.
The molecular sieve catalysts comprise a separately synthesized
framework which is modified by contact with an oxygenated chelating
agent. Substantially any oxygenated chelating agent should operate
in the present invention. Preferred oxygenated chelating agents
comprise functional groups selected from the group consisting of
an anhydride, a plurality of carboxyl groups, a combination of a
hydroxyl group and a carboxyl group, and salts of said functional
groups. Most preferred modifiers are selected from the group consisting
of oxalic acid, maleic acid, maleic anhydride, glutaric acid, glutaric
anhydride, adipic acid, EDTA, o-phthalic acid, o-phthalic anhydride,
terephthalic acid, salycic acid, tartaric acid, salts thereof, and
combinations thereof.
The framework of the molecular sieve catalyst may be modified using
a number of different methods that are readily recognized by persons
of ordinary skill in the art. Examples are static methods, batch
methods, continuous methods, semi-continuous methods, and others.
Given the particular method to be used and the molecular sieve to
be treated, persons of ordinary skill in the art would be able to
determine the optimal time and temperature to be used at a given
concentration of agent and pressure.
In a preferred method, a suitable modifier should be dissolved
in a suitable solvent. The solvent may be water, an organic solvent,
or a mixed solvent, the only limitation being that the modifier
must be soluble in the solvent under the treatment conditions. Supercritical
states of certain solvents, such as water, also may be used, subject
to the same limitation. The pH of the solution should be maintained
in the range of from about 12 to about 1. When water is used as
the primary solvent, it is advantageous to have a pH value smaller
than 7 and larger than about 2.
The solution should be stirred with the separately synthesized
microporous framework for a period of time in the range of from
about 1 minute to about 24 hours at a temperature in the range of
from about 0.degree. C. to about 80.degree. C., preferably in the
range of from about 2 hours to about 24 hours at room temperature.
If a supercritical solvent is used, the temperature and pressure
must be sufficient to maintain such a state. The amount of stirring
time required will depend upon the modifier.
The solid phase should be isolated using any suitable means, preferably
by centrifugation or drying. If centrifugation is used, the residual
solid should be washed, preferably with distilled water, and dried.
Drying at a temperature in the range of from about 90.degree. C.
to about 150.degree. C., preferably at about 100.degree. C., for
a time in the range of from about 1 hour to about 10 hours, preferably
about 2 hours, should be sufficient. If drying is used, the resulting
mixture should be dried for an amount of time sufficient to produce
a dry powder. In a preferred embodiment, the mixture is dried at
a temperature of about 110.degree. C. for about two hours.
The resulting powder should be calcined for a period of time in
the range of from about 2 hours to about 48 hours, preferably for
about 16 hours, at a temperature in the range of from about 300.degree.
C. to about 800.degree. C., preferably in the range of from about
350.degree. C. to about 650.degree. C., most preferably in the range
of from about 550.degree. C. to about 650.degree. C. The resulting
powder may be pressed into pellets and then crushed and sieved to
a mesh size preferably in the range of from about 14 to about 20.
In a preferred embodiment, the pellets are formed by application
of about 137.89521 MPa (20000 psi) of pressure.
The process for converting oxygenates to olefins employs an organic
starting material (feedstock) preferably comprising "oxygenates."
As used herein, the term "oxygenates" is defined to include,
but is not necessarily limited to aliphatic alcohols, ethers, carbonyl
compounds (aldehydes, ketones, carboxylic acids, carbonates, and
the like), and also compounds containing hetero-atoms, such as,
halides, mercaptans, sulfides, amines, and mixtures thereof The
aliphatic moiety preferably should contain in the range of from
about 1 to about 10 carbon atoms and more preferably in the range
of from about 1 to about 4 carbon atoms. Representative oxygenates
include, but are not necessarily limited to, lower straight chain
or branched aliphatic alcohols, their unsaturated counterparts,
and their nitrogen, halogen and sulfur analogues. Examples of suitable
compounds include, but are not necessarily limited to: methanol;
ethanol; n-propanol; isopropanol; C.sub.4 -C.sub.10 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; n-alkyl amines, n-alkyl halides,
n-alkyl sulfides having n-alkyl groups of comprising the range of
from about 3 to about 10 carbon atoms; and mixtures thereof As used
herein, the term "oxygenate" 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.
Preferably, the oxygenate feedstock should be contacted in the
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. Alternately, the process may be carried
out in a liquid or a mixed vapor/liquid phase. When the process
is carried out in the liquid phase or a mixed vapor/liquid phase,
different conversions and selectivities of feedstock-to-product
may result depending upon the catalyst and reaction conditions.
The temperature employed in the conversion process may vary over
a wide range depending, at least in part, on the selected catalyst.
Although not limited to a particular temperature, best results will
be obtained if the process is conducted at temperatures in the range
of from about 200.degree. C. to about 700.degree. C., preferably
in the range of from about 250.degree. C. to about 600.degree. C.,
and most preferably in the range of from about 300.degree. C. to
about 500.degree. C. Lower temperatures generally result in lower
rates of reaction, and the formation of the desired light olefin
products may become markedly slow. However, at higher temperatures,
the process may not form an optimum amount of light olefin products,
and the coking rate may become too high.
Light olefin products will form--although not necessarily in optimum
amounts--at a wide range of pressures, including but not limited
to autogeneous pressures and pressures in the range of from about
0.1 kPa to about 100 MPa,. A preferred pressure is in the range
of from about 6.9 kPa to about 34 MPa, most preferably in the range
of from about 48 kPa to about 0.34 MPa. The foregoing pressures
are exclusive of diluent, if any is present, and refer to the partial
pressure of the feedstock as it relates to oxygenate compounds and/or
mixtures thereof. Pressures outside of the stated ranges may be
used and are not excluded from the scope of the invention. Lower
and upper extremes of pressure may adversely affect selectivity,
conversion, coking rate, and/or reaction rate; however, light olefins
such as ethylene still may form.
The process should be continued for a period of time sufficient
to produce the desired olefin products. The reaction cycle time
may vary from tenths of seconds to a number of hours. The reaction
cycle time is largely determined by the reaction temperature, the
pressure, the catalyst selected, the weight hourly space velocity,
the phase (liquid or vapor), and the selected process design characteristics.
A wide range of weight hourly space velocities (WHSV), defined
as weight feed per hour per weight of catalyst, for the feedstock
will function in the present invention. The WHSV generally should
be in the range of from about 0.01 hr.sup.-1 to about 5000 hr.sup.-1
preferably in the range of from about 0.1 hr.sup.-1 to about 2000
hr.sup.-1 and most preferably in the range of from about 1 hr.sup.-1
to about 1000 hr.sup.-1. The catalyst may contain other materials
which act as inerts, fillers, or binders; therefore, the WHSV is
calculated on the weight basis of oxygenate and catalyst.
One or more diluents may be fed to the reaction zone with the oxygenates,
such that the total feed mixture comprises diluent in a range of
from about 1 mol % and about 99 mol %. Diluents which may be employed
in the process include, but are not necessarily limited to, helium,
argon, nitrogen, carbon monoxide, carbon dioxide, hydrogen, water,
paraffins, other hydrocarbons (such as methane), aromatic compounds,
and mixtures thereof. Preferred diluents are water and nitrogen.
A preferred embodiment of a reactor system for the present invention
is a circulating fluid bed reactor with continuous regeneration,
similar to a modem fluid catalytic cracker. Fixed beds can be used,
but are not ideal for the process because oxygenate to olefin conversion
is a highly exothermic process which requires several stages with
intercoolers or other cooling devices. The reaction also results
in a high pressure drop due to the production of low pressure, low
density gas. Moving beds also may be used.
The invention will be better understood with reference to the following
examples which are intended to illustrate, but not to limit the
present invention. |